Companies are increasingly using global alliances to accelerate their international expansion. This paper identifies the critical factors that influence how the stock market will react to news of this type of alliance. We argue that abnormal returns on such news depend on the degree to which partners fill gaps in their international networks. We also argue that these alliances should be designed to take advantage of these complementarities in a well-defined geographical area. An empirical analysis of the stock market reaction to 72 global alliances by listed Spanish companies confirmed the importance of complementarity and leverage in explaining abnormal returns, and the irrelevance of the region of origin of the partner.