Computer simulations provide environments enabling exploratory learning. Research has shown that these types of learning environments are promising applications of computer assisted learning but also that they introduce complex learning settings, involving a large number of learning processes. This article reports on an instrument for supporting one of these learning processes: stating hypotheses. The resulting instrument, an hypothesis scratchpad, was designed on the basis of a conceptual representation of the simulation model and tested in an experimental study. In this study three versions of the scratchpad, varying in structure, were compared. It was found that support offered for identifying variables, in the form of a selection list, is relatively successful: students who used this list were better in differentiating different types of variables. For identifying relations, a selection list of relations offered to the students proved unhelpful in finding accurate relations: students using this list stated their hypotheses mainly at a very global level. The research reported was conducted in the project SIMULATE. SIMULATE was part of SAFE, a R&D project partially funded by the CEC under contract D1014 within the Exploratory Action of the DELTA programme. The work of SIMULATE is continued in the DELTA main phase project SMISLE.