The purpose of this study was to assess the effects of the instruction method "Thinking Actively in an Academic Context" (TAAC) in the thinking skills of 6th grade students in primary education. The sample consisted of 58 subjects, aged between 11- and 13-years-of-age, 27 in the experimental group and 31 in the control group. A pre-test intervention-post-test design was employed, and the assessment instruments were administered before and after the intervention to measure variables related to thinking and academic achievement. The eight-phase instruction method was administered to all the didactic units of the syllabus-content of environmental knowledge, language and mathematics during one academic course. The stages of the method allow the conjoint teaching of thinking skills and the syllabus-content. The results of the analysis of variance suggest that the intervention produced a significant improvement in both thinking skills and academic achievement, and indicated also that the method produced different effects in reasoning as a function of gender. (Contains 3 tables.)