Mobile adsorption on heterogeneous surfaces at low coverage is modeled via a gas-solid virial expansion, following the ideas of the generalized Gaussian model(6,12) to incorporate energetic topography effects. Adsorbate molecules interact among them via Lennard-Jones interactions. Model predictions are compared to Monte Carlo simulations of adsorption on heterogeneous solids obtained by doping a pure crystalline solid with different concentrations of impurities. Energetic topography effects are shown to be important and they are correctly predicted by the model at low coverage.