The paper presents the thermodynamic analysis of the process of etherification of light FCC gasoline with methanol. In the model presented, it is assumed that the chemical equilibria are reached only for the reactions of methanol with C4-C7 olefins having the double bond on a tertiary carbon atom and the reactions of isomerization of olefins and ethers do not take place. Thermodynamic data which were not available from tables (the Gibbs free energies of formation of C7 olefins and C6-C8 ethers) were calculated using two essentially different group contribution methods (the Benson method and the Yoneda method) to investigate the influence of the data obtained this way on the calculated equilibrium compositions. The nonideality of the liquid phase was described by the modified UNIFAC method and by the ASOG method. In both cases very similar equilibrium compositions were obtained. The parameters were specified for which the system begins to be in the simultaneous chemical and phase (vapor-liquid) equilibrium.