["The RAND Journal of Economics, EarlyView. ", "\nABSTRACT\nWe study a long‐horizon, oligopolistic market with random shocks to demand that can be arbitraged by two storage operators with finite capacity. This problem applies to any storable commodity—that is, most commodities. Because the arbitrage spread is so sensitive to market power, storage operators face strong incentives to restrain quantities by tacitly colluding. This cooperation takes new forms thanks to the multiplicity of actions they must take: selling, buying, or both. We construct payoff‐maximizing equilibria of this stochastic game, and uncover a new form of Partial Cooperation that trades off quantities and delay. While collusive, Partial Cooperation delivers higher consumer surplus thanks to the market power effect. Head‐on competition is not always an equilibrium of the long‐horizon game when market power becomes large enough. We draw implications for policy and suggest poorly competitive storage is a negative externality to the development of the underlying commodity—for example, renewable energy."]