Use of natural language to represent behaviour-change theories has resulted in lack of clarity and consistency, hindering comparison, integration, development and use. This paper describes development of a formal system for representing behaviour-change theories that aims to improve clarity and consistency. A given theory is represented in terms of (1) its component constructs (for example, 'self-efficacy', 'perceived threat' or 'subjective norm'), which are labelled and defined, and (2) relationships between pairs of constructs, which may be causal, structural or semantic. This formalism appears adequate to represent five commonly used theories (health belief model, information-motivation-behavioural skill model, social cognitive theory, theory of planned behaviour and the trans-theoretical model). Theory authors and experts judged that the system was able to capture the main propositions of the theories. Following this proof of concept, the next step is to assess how far the system can be applied to other theories of behaviour change.