More and more French businesses are deciding to introduce an “environmental management system”, i.e. a system to measure and reduce their environmental impact, and then certify their compliance with the ISO 14001 standard. This article aims to assess the environmental effectiveness of certification using a sample of several thousand manufacturing firms. A range of environmental pressures has been analysed: emissions of volatile organic compounds and carbon dioxide, fuel and total energy consumption, and water intake. The data covers several years, from 2001 or 2003 until 2005, making it possible to check the unobserved heterogeneity in the econometric estimation. The results suggest that certified firms focus their efforts on measures likely to reduce not only their negative impact on the environment but also their production costs. Moreover, ISO 14001 certification is invariably accompanied by an immediate reduction in water intake of roughly 8%. A gradual decrease in carbon dioxide emissions and fuel consumption then follows. Finally, after an initial rise in electricity consumption, energy consumption falls by 1% every year. Certification does not, however, appear to have a significant impact on volatile organic compound emissions.