The purpose of this paper is to compare some Brazilian energy and CO, emission scenarios in order to verify how far model structures influence findings and decisions. Two kinds of comparison were designed. The first one is between two scenarios drawn up for the same model, assessing the impact of energy and environmental policies in both quantitative and qualitative terms. The second comparison is based on the results of another model prepared in the early 1980s. Although the model structures are very different (optimisation versus simulation) the relative aggregate results for energy consumption and carbon dioxide emissions are relatively close. The differences al e due largely to structural changes in the Brazilian economy not anticipated at that time. However, when the model is broken down to display the specific needs, priorities and constraints of Brazil, the results show appreciable qualitative differences. This leads to the conclusion that an analysis of the functioning of the economy is important for drawing up or adapting models. Consequently, it is believed that alternative energy policies designed to mitigate environmental impacts could be assessed more effectively.