The authors extend findings from qualitative research on the effects of action research by reporting two linked quantitative studies (N = 80 and 105). They found that teachers who participated in collaborative action research experienced statistically significant improvements in attitudes to educational research and teacher efficacy. The pre-post changes were robust across conditions of teacher gender, career stage, and qualifications. Teachers benefited more if they (i) recognized the importance of the data analysis and process reflection stages of action research; (ii) participated in action research that was rigorous and/or led to changes in their conceptual understanding; (iii) worked in schools that fostered professional learning; and (iv) had participated in research activities prior to these action research studies. (Contains 2 notes, 7 tables and 3 figures.)