In this paper we provide a theoretical foundation for the Porter hypothesis in a context of quality competition. We use a duopoly model of vertical product differentiation where firms simultaneously choose the environmental quality of the good they produce (which can be either high or low) and, afterwards, engage in price competition. In this simple setting, we show that a Nash equilibrium of the game with low quality could be Pareto dominated by another strategy profile in which both firms produce the high environmental quality good. We then show how, in this case, the introduction of a penalty to any firm that produces the low environmental quality can result in an increase in both firms' profits. The impact of the policy on consumers depends on the effect of a quality shift on the cost structure of firms.