diff --git a/include/loki/details/pddl/axiom.hpp b/include/loki/details/pddl/axiom.hpp
index 0d837bd8..e7da5a70 100644
--- a/include/loki/details/pddl/axiom.hpp
+++ b/include/loki/details/pddl/axiom.hpp
@@ -32,8 +32,9 @@ class AxiomImpl : public Base<AxiomImpl>
     std::string m_derived_predicate_name;
     ParameterList m_parameters;
     Condition m_condition;
+    size_t m_num_parameters_to_ground_head;
 
-    AxiomImpl(size_t identifier, std::string derived_predicate_name, ParameterList parameters, Condition condition);
+    AxiomImpl(size_t identifier, std::string derived_predicate_name, ParameterList parameters, Condition condition, size_t num_parameters_to_ground_head);
 
     // Give access to the constructor.
     friend class PDDLFactory<AxiomImpl, Hash<AxiomImpl*>, EqualTo<AxiomImpl*>>;
@@ -50,6 +51,7 @@ class AxiomImpl : public Base<AxiomImpl>
     const std::string& get_derived_predicate_name() const;
     const ParameterList& get_parameters() const;
     const Condition& get_condition() const;
+    size_t get_num_parameters_to_ground_head() const;
 };
 
 }
diff --git a/include/loki/details/pddl/factories.hpp b/include/loki/details/pddl/factories.hpp
index 58e80a09..0c274fa8 100644
--- a/include/loki/details/pddl/factories.hpp
+++ b/include/loki/details/pddl/factories.hpp
@@ -264,9 +264,9 @@ class PDDLFactories
         return actions.get_or_create<ActionImpl>(std::move(name), std::move(original_arity), std::move(parameters), std::move(condition), std::move(effect));
     }
 
-    Axiom get_or_create_axiom(std::string derived_predicate_name, ParameterList parameters, Condition condition)
+    Axiom get_or_create_axiom(std::string derived_predicate_name, ParameterList parameters, Condition condition, size_t num_parameters_to_ground_head)
     {
-        return axioms.get_or_create<AxiomImpl>(std::move(derived_predicate_name), std::move(parameters), std::move(condition));
+        return axioms.get_or_create<AxiomImpl>(std::move(derived_predicate_name), std::move(parameters), std::move(condition), num_parameters_to_ground_head);
     }
 
     OptimizationMetric get_or_create_optimization_metric(OptimizationMetricEnum metric, FunctionExpression function_expression)
diff --git a/src/pddl/axiom.cpp b/src/pddl/axiom.cpp
index 76a09bfb..2ca24e71 100644
--- a/src/pddl/axiom.cpp
+++ b/src/pddl/axiom.cpp
@@ -28,11 +28,16 @@
 
 namespace loki
 {
-AxiomImpl::AxiomImpl(size_t identifier, std::string derived_predicate_name, ParameterList parameters, Condition condition) :
+AxiomImpl::AxiomImpl(size_t identifier,
+                     std::string derived_predicate_name,
+                     ParameterList parameters,
+                     Condition condition,
+                     size_t num_parameters_to_ground_head) :
     Base(identifier),
     m_derived_predicate_name(std::move(derived_predicate_name)),
     m_parameters(std::move(parameters)),
-    m_condition(std::move(condition))
+    m_condition(std::move(condition)),
+    m_num_parameters_to_ground_head(num_parameters_to_ground_head)
 {
 }
 
@@ -68,4 +73,6 @@ const Condition& AxiomImpl::get_condition() const { return m_condition; }
 
 const ParameterList& AxiomImpl::get_parameters() const { return m_parameters; }
 
+size_t AxiomImpl::get_num_parameters_to_ground_head() const { return m_num_parameters_to_ground_head; }
+
 }
diff --git a/src/pddl/parser/structure.cpp b/src/pddl/parser/structure.cpp
index 36a2d40e..e374435a 100644
--- a/src/pddl/parser/structure.cpp
+++ b/src/pddl/parser/structure.cpp
@@ -113,13 +113,14 @@ Axiom parse(const ast::Axiom& node, Context& context)
         variables.erase(axiom_parameter->get_variable());
     }
     // Turn free variables not mentioned in the parameter list into parameters
+    // Those parameters must be appended to the parameter list
     for (const auto variable : variables)
     {
         const auto base_types = TypeList { context.factories.get_or_create_type("object", TypeList {}) };
 
         parameters.push_back(context.factories.get_or_create_parameter(variable, base_types));
     }
-    const auto axiom = context.factories.get_or_create_axiom(predicate_name, parameters, condition);
+    const auto axiom = context.factories.get_or_create_axiom(predicate_name, parameters, condition, parameters.size());
     context.positions.push_back(axiom, node);
     return axiom;
 }