In this paper we develop a simple equilibrium business-cycle model for an economy with both clean-and dirty-producing plants. We derive that the optimal timing of cleaning the production process is during a slowdown of the economy. Due to external effects and market failures the timing of pollutionabatement 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 U.S.A. It appears that for more than 25 percent of the sectors pollution abatement investments show counter-cyclical behaviour,while in only one sector these investments are pro-cyclical.