Formal specification languages can improve the development of knowledge-based systems (KBS), but several problems limit their usefulness. (ML)2, a formal language based on the knowledge models used in the CommonKADS KBS development method, avoids many of these problems. (ML)2 specifically aims at formalizing the CommonKADS expertise model. To analyze (ML)2 usability, a set of evaluation criteria was designed. A small case study was then performed, constructing and expertise model in (ML)2, to test and refine this criteria.