Chemisorption of atomic oxygen and sulfur on Pt(111) has been studied by means of the ab initio cluster model approach. For both adsorbates, we consider chemisorption on the threefold open site of the Pt(111) surface which is represented by a Pt-25 cluster model having 12 atoms in the first layer, six in the second and seven in the third. The ab initio molecular orbital Hartree-Fock method and explicitly correlated wavefunctions are used to obtain a reliable estimate of the interaction energy, vibrational frequency for the normal mode of the adsorbate above the surface, and the equilibrium geometry. The chemisorption bond is analyzed using different theoretical techniques including the constrained space orbital variation, CSOV, method, the analysis of dipole moment curves and the use of projection operators. The influence of electronic correlation effects is analyzed using multireference configuration interaction, MRCI, techniques and also, by using post-Hartree-Fock correlation functional.