We analyze how different rules for exiting an alliance affect the formation of strategic alliances. We adopt the concept of contractual stability to predict the alliances that are going to emerge in the long run. We find that any asymmetric alliance structure consisting of two alliances is contractually stable under the unanimity decision rule. In addition, the grand alliance which is the efficient structure is stable. If we allow for side payments to compensate former partners, then some less efficient structures that were stable without side payments are no longer stable. Moreover, we show that the stability of alliances under the unanimity rule to exit is robust to the type of firms, myopic or farsighted. Finally, there is no contractually stable alliance structure under the simple majority decision rule.