diff --git a/src/composition/private/composition.cpp b/src/composition/private/composition.cpp
index 0fefc7b..822d67d 100644
--- a/src/composition/private/composition.cpp
+++ b/src/composition/private/composition.cpp
@@ -459,4 +459,25 @@ Composition Composition::subset(const std::vector<std::string>& symbols, std::st
         }
     }
     return subsetComposition;
+}
+
+void Composition::setCompositionMode(bool massFracMode) {
+    if (!m_finalized) {
+        LOG_ERROR(m_logger, "Composition has not been finalized. Mode cannot be set unless composition is finalized.");
+        throw std::runtime_error("Composition has not been finalized (Consider running .finalize()). The mode cannot be set unless the composition is finalized.");
+    }
+
+    bool okay = true;
+    for (auto& [_, entry] : m_compositions) {
+        if (massFracMode) {
+            okay = entry.setMassFracMode(m_meanParticleMass);
+        } else {
+            okay = entry.setNumberFracMode(m_specificNumberDensity);
+        }
+        if (!okay) {
+            LOG_ERROR(m_logger, "Composition mode could not be set.");
+            throw std::runtime_error("Composition mode could not be set.");
+        }
+    }
+    m_massFracMode = massFracMode;
 }
\ No newline at end of file
diff --git a/src/composition/public/composition.h b/src/composition/public/composition.h
index 7335efb..6893ab7 100644
--- a/src/composition/public/composition.h
+++ b/src/composition/public/composition.h
@@ -403,6 +403,12 @@ namespace composition{
          */
         Composition subset(const std::vector<std::string>& symbols, std::string method="norm") const;
 
+        /**
+        * @brief Sets the composition mode.
+        * @param massFracMode True if mass fraction mode, false if number fraction mode.
+        */
+        void setCompositionMode(bool massFracMode);
+
         /**
          * @brief Overloaded output stream operator for Composition.
          * @param os The output stream.
diff --git a/tests/composition/compositionTest.cpp b/tests/composition/compositionTest.cpp
index 5d6da06..4dc9ae6 100644
--- a/tests/composition/compositionTest.cpp
+++ b/tests/composition/compositionTest.cpp
@@ -1,4 +1,5 @@
 #include <gtest/gtest.h>
+#include <stdexcept>
 #include <string>
 #include <algorithm>
 
@@ -138,4 +139,26 @@ TEST_F(compositionTest, getComposition) {
     EXPECT_DOUBLE_EQ(compositionEntry.first.mass_fraction(), 0.6);
     EXPECT_DOUBLE_EQ(compositionEntry.second.meanParticleMass, 1.4382769310381101);
     EXPECT_DOUBLE_EQ(compositionEntry.second.specificNumberDensity, 1.0/1.4382769310381101);
+}
+
+TEST_F(compositionTest, setCompositionMode) {
+    Config::getInstance().loadConfig(EXAMPLE_FILENAME);
+    composition::Composition comp;
+    comp.registerSymbol("H-1");
+    comp.registerSymbol("He-4");
+    comp.setMassFraction("H-1", 0.6);
+    comp.setMassFraction("He-4", 0.4);
+    EXPECT_NO_THROW(comp.finalize());
+
+    EXPECT_DOUBLE_EQ(comp.getMassFraction("H-1"), 0.6);
+    EXPECT_DOUBLE_EQ(comp.getMassFraction("He-4"), 0.4);
+
+    EXPECT_NO_THROW(comp.setCompositionMode(false));
+
+    EXPECT_NO_THROW(comp.setNumberFraction("H-1", 0.9));
+    EXPECT_NO_THROW(comp.setNumberFraction("He-4", 0.1));
+
+    EXPECT_THROW(comp.setCompositionMode(true), std::runtime_error);
+    EXPECT_NO_THROW(comp.finalize());
+    EXPECT_NO_THROW(comp.setCompositionMode(true));
 }
\ No newline at end of file