Objective: Formal definition of composition assessment metrics for CBSE, using an extension of the CORBA Component Model metamodel as the ontology for describing component assemblies. Method: Representation of a component assembly as an instantiation of the extended CORBA Component Model metamodel. The resulting meta-objects diagram can then be traversed using Object Constraint Language clauses. These clauses are a formal and executable definition of the metrics that can be used to assess quality attributes from the assembly and its constituent components. Results: Formal definition of context-dependent metrics that cover the different composition mechanisms provided by the CORBA Component Model and can be used to compare alternative component assemblies; a metamodel extension to capture the topology of component assemblies. Conclusion: Providing a formal and executable definition of metrics for CORBA component assemblies is an enabling precondition to allow for independent scrutiny o...