We present a mean field theory of adsorption for fluids in heterogeneous solids such as silica gel. The theory is based on a molecular model which treats the silica gel as a matrix of particles with a predefined structure. Under certain well defined conditions, adsorption of fluids in such an adsorbent can be treated similarly to the problem of the solubility of adsorbate molecules in an equilibrium fluid mixture of adsorbate molecules and matrix particles. We investigate the simplest mean field approximation for this problem. Comparison is made with Monte Carlo simulations for single component and binary mixture adsorption of argon and methane in silica gel. For these systems accurate prediction of the adsorption equilibrium can be obtained by fitting the adsorbate-adsorbate parameters in the mean field theory to the bulk equation of state and the adsorbate-adsorbent parameters to Henry’s law constant for adsorption. Comparison with experiment for ethane-methane mixtures is also presented. In this case it was necessary to adjust the adsorbate-adsorbent interaction parameter away from Henry’s law value in order to provide. a reasonable fit of the pure component isotherm over the entire pressure range. Good predictions of binary adsorption equilibria can then be obtained from the pure component parameters.