This paper reports the results of an action research project on improving students' learning from international service-learning. Participants were two consecutive cohorts of university students enrolled in nine international service-learning projects. Mixed-method findings from the first cohort reveal significant increases in their global competence and intercultural effectiveness but not their social responsibility or global civic engagement. Targeted improvement actions were planned and implemented accordingly for the subsequent cohort, resulting in significant increases in their social responsibility in addition to global competence and intercultural effectiveness scores. No significant increase, however, was found in students' global civic engagement. The findings suggest that action research can be an effective strategy to improve students' global citizenship and intercultural effectiveness development from international service-learning, and that explicit and intentional elements are needed in the learning and reflective activities to develop these attributes. Implications for practice and future research are discussed.