Based on a random binding lattice gas model an analytical theory for HCl adsorption on ice is developed, taking into account the surface heterogeneity (O-, OH-sites and point-like defects). The heterogeneity is shown to be one of possible reasons for a dependence of the adsorption energy DeltaG on the HCl pressure. The HCl chemical diffusion coefficient is shown to be quite sensitive to the pressure variations. The defects may increase DeltaG up to experimental results (33-46 kJ/ mol) but the coverage is limited to the fraction of the defect sites, such that the approach is restricted to a submonolayer regime.