Alliances of buyers to negotiate input prices with suppliers are commonplace. Using pre- and post-alliances data on household purchases of bottled water, I develop a structural model of bilateral oligopoly to estimate the effects of three alliances formed by retailers on their bargaining power vis-à-vis manufacturers and retail prices paid by consumers. Results provide evidence of a countervailing buyer power effect that reduces retail prices by roughly 7%. Exploring determinants of buyer power, I find that changes in retailers' bargaining ability play an important role in the countervailing force exerted by the alliances which, otherwise, would have not been profitable.