For the first time Wannier functions are generated for systems with two-dimensional periodicity (slabs) and used for the surface charge distribution analysis. It is suggested to use bulk Wannier functions to estimate the slab thickness, needed for the convergence of the results. The (001)-surface of MgO and (110)-surface of TiO₂ crystals are studied. The population analysis of the (001)-surface slab of MgO, based on Wannier-type atomic orbitals (WTAOs), demonstrates a purely ionic nature of chemical bonding on the studied crystalline surface. For TiO₂ the WTAOs method gives the mixed ionic-covalent type of chemical bonding on the (110)-surface with the covalent component at the surface being larger than inside the slab.