As part of its waste matter prevention policy, the Dutch government has tried over the past few years to stimulate pollution prevention in firms by means of so-called stimulation and learning projects. To be able to determine the effectiveness of future policies, an extensive evaluation study was performed in 1994 on the pollution prevention projects which had been realized over the past few years. This study* was carried out by the Centre for Clean Technology and Environmental Policy of the University of Twente in Enschada, the Netherlands, commissioned by the Ministry of Housing, Physical Planning and Environment, Waste Matter Board. The aim of the study was to determine the level of success of pollution prevention projects in relation to different circumstances in the implementation of pollution prevention projects in firms. This would provide information which may contribute to the successful setting up of new projects. By `successful¿ we mean that they may build on the success formulas of projects completed earlier, consisting of specific project features and other conditions which proved to be of influence. The central research question of the study was: which features make a pollution prevention project efficacious, effective and efficient? This article deals with the structure and results of this study and gives insight into: (1)the results of pollution prevention projects in the Netherlands; and (2)the features of pollution prevention projects which proved to be either more or less successful.