The purpose of this study is to examine the effects of strategic alliances on participating firms' productivity. In particular, this study develops hypotheses regarding the effects on firm productivity of various alliance conditions, including the number of alliances, the level of cooperation between partners, equity investment in partners, and relative partner size. Based on panel data from 22 major international airlines that formed international alliances during the period 1986-95, we find that alliances make a significant contribution to productivity gains. Further, it is found that the level of cooperation between partners increases the productivity gains, whereas equity investment in partners and relative partner size do not. These results suggest that firms should attempt to increase the level of cooperation to achieve higher productivity gains.