Why do SMEs invest in environmental measures? From the literature we know that most SMEs are rather slow in adopting these sustainable measures, but then the following question arises: why are other SMEs fast in this respect? From our research, it becomes clear that improving the working conditions is the most important reason why these fast-going SMEs invest in environmental measures, as this probably improves their employees' motivation and performance. This explanation connects 'planet' and 'people' with each other, and does not exclude 'profit', which completes the traditional sustainability circle. Copyright © 2006 John Wiley & Sons, Ltd and ERP Environment.