This paper analyzes the long-term investment decisions of firms that are regulated by an emissions tax and that perceive a degree of market power in their respective output markets. Firms invest in abatement equipment that is fixed over the medium term (e.g., buying a new generator). This paper focuses on environmental regulation with and without commitment. In the commitment case, the government announces a long-run tax on emissions, and firms decide upon their investment levels. In the no-commitment case, the regulator announces a tax level and is free to modify it once firms have invested. This paper considers differentiated product goods and determines whether no-commitment regulation leads to more lenient or more stringent regulation than regulation with commitment.