If barriers between two power markets are eliminated, what might happen to competition and prices? And who benefits? In the case of the Belgian and Dutch markets, market coupling would permit more efficient use of transmission by improving access to the Belgian market, by counting only net flows against interface limits, and by eliminating mismatches in timing of interface auctions and energy spot markets. We estimate the benefits associated with the first two of these impacts using a transmission-constrained Cournot model. Social surplus improvements on the order of 10(8) Euro/year are projected, unless market coupling encourages the largest producer in the region to switch from price-taking in Belgium to a Cournot strategy due to a perceived diminished threat of regulatory intervention. Whether Dutch consumers would benefit also depends on that company's behavior. The results illustrate how large-scale oligopoly models can be used to assess changes in market designs.