diff --git a/include/fintamath/core/MathObjectUtils.hpp b/include/fintamath/core/MathObjectUtils.hpp
index 80a6f3185..077aa95ed 100644
--- a/include/fintamath/core/MathObjectUtils.hpp
+++ b/include/fintamath/core/MathObjectUtils.hpp
@@ -129,4 +129,12 @@ inline std::shared_ptr<To> cast(const std::shared_ptr<From> &from) noexcept {
   return std::static_pointer_cast<To>(from);
 }
 
-}
\ No newline at end of file
+template <typename Comparator>
+struct ToStringComparator {
+  template <typename T>
+  bool operator()(const T &lhs, const T &rhs) const noexcept {
+    return Comparator{}(lhs.toString(), rhs.toString());
+  }
+};
+
+}
diff --git a/include/fintamath/expressions/IExpression.hpp b/include/fintamath/expressions/IExpression.hpp
index dfa5e44ff..f7fb2bd48 100644
--- a/include/fintamath/expressions/IExpression.hpp
+++ b/include/fintamath/expressions/IExpression.hpp
@@ -2,12 +2,14 @@
 
 #include <concepts>
 #include <memory>
+#include <set>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "fintamath/core/IMathObject.hpp"
 #include "fintamath/core/MathObjectClass.hpp"
+#include "fintamath/core/MathObjectUtils.hpp"
 #include "fintamath/core/Parser.hpp"
 #include "fintamath/functions/FunctionArguments.hpp"
 #include "fintamath/functions/IFunction.hpp"
@@ -20,6 +22,9 @@ namespace fintamath {
 class IExpression : public IMathObject {
   FINTAMATH_PARENT_CLASS_BODY(IExpression, IMathObject)
 
+public:
+  using VariableSet = std::set<Variable, ToStringComparator<std::less<>>>;
+
 public:
   virtual const std::shared_ptr<IFunction> &getFunction() const = 0;
 
@@ -27,7 +32,7 @@ class IExpression : public IMathObject {
 
   virtual void setChildren(const ArgumentPtrVector &childVect) = 0;
 
-  std::vector<Variable> getVariables() const;
+  VariableSet getVariables() const;
 
   virtual void setVariables(const std::vector<std::pair<Variable, ArgumentPtr>> &varsToVals);
 
diff --git a/src/fintamath/expressions/IExpression.cpp b/src/fintamath/expressions/IExpression.cpp
index 9a863dac0..90bc34430 100644
--- a/src/fintamath/expressions/IExpression.cpp
+++ b/src/fintamath/expressions/IExpression.cpp
@@ -29,23 +29,19 @@ FINTAMATH_PARENT_CLASS_IMPLEMENTATION(IExpression)
 
 using namespace detail;
 
-std::vector<Variable> IExpression::getVariables() const {
-  std::vector<Variable> vars;
+IExpression::VariableSet IExpression::getVariables() const {
+  VariableSet vars;
 
   for (const auto &child : getChildren()) {
     if (auto var = cast<Variable>(child)) {
-      vars.emplace_back(*var);
+      vars.emplace(*var);
     }
     else if (const auto childExpr = cast<IExpression>(child)) {
-      std::vector<Variable> childVars = childExpr->getVariables();
-      vars.insert(vars.end(), childVars.begin(), childVars.end());
+      VariableSet childVars = childExpr->getVariables();
+      vars.insert(childVars.begin(), childVars.end());
     }
   }
 
-  std::ranges::sort(vars, std::less{}, &Variable::toString);
-  auto unique = std::ranges::unique(vars);
-  vars.erase(unique.begin(), unique.end());
-
   return vars;
 }
 
diff --git a/src/fintamath/expressions/functions/ExpressionFunctionSolve.cpp b/src/fintamath/expressions/functions/ExpressionFunctionSolve.cpp
index 361b48f94..3ba64b2f1 100644
--- a/src/fintamath/expressions/functions/ExpressionFunctionSolve.cpp
+++ b/src/fintamath/expressions/functions/ExpressionFunctionSolve.cpp
@@ -44,7 +44,7 @@ Expression solve(const Expression &rhs) {
 
   // TODO: remove this if when inequalities will be implemented
   if (!is<Eqv>(compExpr->getFunction())) {
-    const auto var = cast<Variable>(compExpr->getVariables().front());
+    const auto var = cast<Variable>(*compExpr->getVariables().begin());
     const ArgumentPtrVector powerRate = getPolynomCoefficients(compExpr->getChildren().front(), var);
 
     if (powerRate.size() == 2) {
@@ -55,7 +55,7 @@ Expression solve(const Expression &rhs) {
     return rhs;
   }
 
-  const auto var = cast<Variable>(compExpr->getVariables().front());
+  const auto var = cast<Variable>(*compExpr->getVariables().begin());
   const ArgumentPtrVector powerRates = getPolynomCoefficients(compExpr->getChildren().front(), var);
   ArgumentPtrVector roots;
 
diff --git a/tests/src/core/MathObjectUtilsTests.cpp b/tests/src/core/MathObjectUtilsTests.cpp
index de79b21ed..b76df3b2e 100644
--- a/tests/src/core/MathObjectUtilsTests.cpp
+++ b/tests/src/core/MathObjectUtilsTests.cpp
@@ -57,3 +57,7 @@ TEST(MathObjectUtilsTests, castTest) {
   EXPECT_TRUE(cast<IArithmetic>(std::const_pointer_cast<const IMathObject>(std::shared_ptr(i.clone()))));
   EXPECT_FALSE(cast<IArithmetic>(std::const_pointer_cast<const IMathObject>(std::shared_ptr(E().clone()))));
 }
+
+TEST(MathObjectUtilsTests, compareByToStringTest) {
+  // TODO: implement
+}
diff --git a/tests/src/expressions/IExpressionTests.cpp b/tests/src/expressions/IExpressionTests.cpp
index b49aac62f..528160b08 100644
--- a/tests/src/expressions/IExpressionTests.cpp
+++ b/tests/src/expressions/IExpressionTests.cpp
@@ -1,8 +1,10 @@
+#include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
 #include "fintamath/expressions/IExpression.hpp"
 
 #include "fintamath/expressions/Expression.hpp"
+#include "fintamath/expressions/ExpressionParser.hpp"
 #include "fintamath/expressions/ExpressionUtils.hpp"
 #include "fintamath/functions/arithmetic/Add.hpp"
 #include "fintamath/functions/other/Factorial.hpp"
@@ -84,11 +86,8 @@ TEST(IExpressionTests, setChildrenTest) {
 }
 
 TEST(IExpressionTests, getVariablesTest) {
-  const auto expr = cast<IExpression>(Expression("x^2+y^2+a").clone());
-  const auto vars = expr->getVariables();
-  EXPECT_EQ(vars[0].toString(), "a");
-  EXPECT_EQ(vars[1].toString(), "x");
-  EXPECT_EQ(vars[2].toString(), "y");
+  const auto expr = cast<IExpression>(parseExpr("x^2+y^2+a"));
+  EXPECT_THAT(expr->getVariables(), testing::ElementsAre(Variable("a"), Variable("x"), Variable("y")));
 
   // TODO: implement more tests
 }