Abtract: In this article, we compare a tax and a standard as environmental tools depending on firms’ R &D strategy and the government’s ability to credibly commit to its policy. We consider a duopoly model where production is polluting and in an effort to mitigate emissions, firms invest in green R &D (in the presence of technological spillovers) either cooperatively or non-cooperatively. We explore two policy games in which the regulator establishes an emission tax or an emission standard either before or after firms engage in R &D. We endogenize both the firms’ R &D strategy and the regulator’s choice of policy instrument. We find that an emission standard is adopted only when firms choose not to cooperate. Conversely, a tax is desirable when firms collaborate in green R &D. Moreover, we expand our framework by offering the opportunity for the regulator to authorize or ban cooperation in green R &D before the firms make their strategic decisions.