In a project carried out for a major French company in the energy sector, we used the data from the Life Cycle Inventory (LCI) of an industrial transformation process and experimented with three external cost evaluation methods. This paper is a presentation of the results obtained. The first part outlines the three external cost evaluation methods and presents the way we used them. The second part presents the results obtained. The third part discusses the relevance of external costs in a corporate decision making context.