Output-based refunding of environmental policy revenues combines a tax on emissions with a subsidy to output. With imperfect competition, subsidies can discourage output underprovision. However, when market shares are significant, endogenous refunding suffers compared to a fixed subsidy. Refunding the emissions tax according to market share reduces the incentive to abate, and marginal abatement costs will not be equalized if market shares differ. In a Cournot duopoly, endogenous refunding leads to higher output, emissions, and possibly costs compared to a fixed rebate program. These results hold whether emission rates are determined simultaneously or strategically in a two-stage model.