This work has as its purpose to evaluate the greenhouse effect gases (GHGs) emissions from ethanol-powered buses, which were introduced in the urban transportation system of the city of São Paulo, in 2007, by means of Project BEST - BioEthanol for Sustainable Transport with the goal of promoting ethanol usage in substitution to diesel in public transportation, aiming at the reduction of atmospheric pollution in great urban centers and at the reduction of global warming. The engine is advanced even for the strict European emission standards, because it meets the EURO 5 specifications and the Enhanced Environmental Vehicle (EEV), besides surpassing the limits imposed by the P-7 stage of PROCONVE. Today, ethanol-powered buses became a reality, because the chassis and the engine, then imported, as well as the additive for the ethanol, are already manufactured in Brazil. Starting from the signature, by initiative of the city hall, of a protocol of intent for the purchase of the first 50 ethanol buses, other actions come following, as 10 more buses were delivered. Besides evaluating GHGs emissions from ethanol-powered buses, this work analyzes the impacts in CO2 emissions due to the gradual substitution of the whole fleet of the city of São Paulo, currently powered by diesel, by ethanol buses. As there are no methodologies approved by the Clean Development Mechanism (CDM) Executive Council, of the Kyoto Protocol, that fully apply to the proposed situation, calculations are executed from equations predicted by the Greenhouse Gas (GHG) Protocol and the Intergovernanmental Panel on Climate Change (IPCC) in order to determine the income generated by the carbon credits.