The present study relates to a research line in didactics of science that focuses on the function that sign systems or semiotic modes have in communicative interactions in science classrooms. The study was conducted by 20 secondary and university science teachers who belong to the same research group LIEC ("Language and Science Teaching") at the Universitat Autonoma de Barcelona (UAB). This group is devoted to the study of the relations between language and science teaching, and is centred on the analysis of textbooks, because these are still the most used "didactical mediators" in science teaching. The study attempted to construct indicators in order to investigate whether textbooks are adequate for didactical intervention. Such an investigation would provide useful information for better strategies both to read in class and to write more innovative textbooks from a "didactical" point of view, i.e., textbooks that are more appropriate to support students' autonomous work in science classrooms. (Contains 1 figure and 2 tables.)