We model competition in an emissions trading system (ETS) as a game between two firms and environmental group. In a previous stage, firms endogenously choose their manufacturing technologies. Our results show that there is an inverted U-shape relationship between how polluting the chosen technology is and the degree of the environmentalists' impure altruism. Firms choose a less polluting technology in the presence of the environmentalists than in their absence only if they are characterised by intermediate degrees of impure altruism.