Background: Every teacher should be able to use curriculum materials to guide instructional design and make reasoned pedagogical decisions about the limitations these resources may have. Objectives : In this paper, we describe and analyse a formative intervention with prospective high school mathematics teachers, aimed at developing their competence of didactic suitability analysis of mathematics textbook lessons. Design: The methodology followed is didactic engineering, furthermore, the content analysis methodology is applied to examine the response protocols of the participants. Setting and Participants : The experience was carried out within the framework of a University Master's Degree in Compulsory Secondary Education and High School; the sample was made of 30 students. Data collection and analysis : We proposed these prospective teachers systematically and critically analyse a lesson on proportionality. The written reports of the lesson suitability produced by 14 work-teams are compared with the a priori analysis carried out by the researchers. Results : The results suggest that the prospective teachers usually make more descriptive and less analytical analyses even while using a guide. The participants did not clearly identify the epistemic deficiencies of the lesson, thus revealing their limited didactic-mathematical knowledge on proportionality and their lack of critical evaluation of the textbook. However, based on the analysis previously conducted, prospective teachers managed to be quite accurate in preparing their proposals for the use of the textbook lesson. Conclusions : In this article we show the interest and usefulness of providing future teachers with a tool to systematically analyse a specific textbook lesson. However, in order for future teachers to acquire the necessary skills in critical analysis of the lesson, it is necessary to reinforce their didactic-mathematical knowledge related to proportionality.