In this paper we analyze the effects of an environmental policy on the diffusion of a clean technology in an economy where firms compete on the output market. We show that the share of adopting firms is non-monotonic with the stringency of the environmental policy, and that the adoption of the clean technology may well increase the pollution level. We also compare the effects of an emission tax and tradable pollution permits on welfare, technology adoption, and pollution level. We show that, depending on the stringency of the policy, either the tax or the permits can yield a higher degree of technology adoption and pollution. Actually, technology adoption and environmental quality may be conflicting in discriminating among the instruments.