In this paper we develop an equilibrium business-cycle model for an economy with both clean and dirty (polluting) plants. We conclude that the best time to implement cleaner production technologies is during a slowdown of the economy. Due to external effects and market failures the timing of pollution abatement investments is not expected to be optimal in the real world. We test the optimality of the timing of those investments with data for Germany, the Netherlands, and the USA. It appears that for more than 25 per cent of the sectors pollution abatement investments show significant counter-cyclical behaviour, while in 10 per cent of the sectors these investments are pro-cyclical.