Permit markets lead polluting firms to purchase abatement goods from an eco‐industry which is often concentrated. This paper studies the consequences of this sort of imperfectly competitive eco‐industry on the equilibrium choices of the competitive polluting firms. It then characterizes the second‐best pollution cap. By comparing this situation to one of perfect competition, we show that Cournot competition on the abatement good market contributes not only to a nonoptimal level of emission reduction but also to a higher permit price, which reduces the production level. These distortions increase with market power, measured by the margin taken by the noncompetitive firms, and suggest a second‐best larger pollution cap.