A molecular field method is used to study a spin-one Ising (lattice-gas) model appropriate for the multilayer adsorption of a two-component gas. The model is used in its simplest form to discribe the phase transition and coadsorption isotherms of the mixture. We found that the phase transition occurs in a narrow interval of the total coverage. The adsorption isotherms depend on the chemical potentials of both components and on the temperature. The density profiles of both components are also presented.