The competence of modelling as part of learning about chemical change is analysed in a sample of 35 secondary students, ages 14-15 years, during their study of a curricular unit on this topic. The teaching approach followed is model based, with frequent use of analogies and mechanical models (fruits and bowls, Lego pieces, balls of plasticine, discs of coloured card, etc.) as mediators between the students' intuitive understanding and school science models. Qualitative and quantitative methods of data analysis were used, acquiring information through portfolios, interviews, the teacher's diary, and audiotapes. The qualitative results allowed a set of 12 dimensions to be defined that were used to characterize and evaluate different aspects of the competence of modelling. The assessment of the students' performance in each of these dimensions by means of a 4-level ordinal rubric allowed the data to be analysed quantitatively. The quantitative results showed the overall set of these dimensions to have construct validity, with 2 sub-constructs standing out: "Working with Models" and "The Nature of Models". The students reached satisfactory levels of competence in both of these sub-constructs, particularly in the latter.