In recent years, the market for fuel has undergone changes due to the expansion of sales of alternative fuels to gasoline, hydrated alcohol and natural gas. Given this scenario, the objective was to define the relevant market for the fuel, gasoline C, hydrous ethanol and compressed natural gas (CNG) in the national market, from January 2002 to December 2008. To this end, we used the cointegration tests, decomposition of the variance of prediction error and Granger causality. The results showed that the three fuels are the same relevant market product. Therefore, any change in the price of one, can generate substitution, so that such changes may have only a transitory nature.