A lattice gas model of mixed adsorption on a heterogeneous surface is considered. The assumption that the differences between the energies of the components on each adsorption site are the same makes this model an ideal mixture. It is shown that mixing at constant spreading pressure in this case is equivalent to that at constant coverage. Interaction between adsorbed molecules induces deviations from ideality, but the excess chemical potentials of the components calculated in the random mixing (mean field) approximation depend linearily upon mole fractions, contrary to the regular bulk solutions which in the same approximation have a quadratic dependence on mole fractions. A new method of describing the experimental data is proposed in which two experimental isotherms are fitted by two model equations with adjustable parameters under the additional constraint that some parameters in the two equations must be equal.