Formal knowledge modelling languages have a number of advantages over informal languages, such as their precise meaning and the possibility to derive propertiesthrough formal proofs. However, theseformal languagesalsosuffer from problems which limit their practical usefulness:they are often not expressive enough to deal with real world applications, formal models are complex and hard to read, and constructing a formal model is a difficult, error prone and expensive process. The goal of the study presentedin this paper is to investigate the usability of one such formal KBS modelling language, called (ML)