The ab initio cluster model approach has been applied to the study of chemisorption of atomic S on Pt(111). Hartree-Fock and explicitly correlated wave functions predict reasonable results for the equilibrium distance perpendicular to the surface and its corresponding vibrational frequency. However, the interaction energy has been shown to be largely affected by electronic correlation effects. Use of MR-MP2 and MRCI wave functions with large reference spaces results in a D-e value which is surprisingly close to the experimental estimate of the interaction energy. All approaches suggest that S is strongly bonded to the Pt(111) surface through covalent interactions but also predict that chemisorbed S exhibits a rather large negative charge. Therefore, it is predicted that S chemisorption will strongly affect the electronic properties of the Pt(111) surface. This situation will be specially valid in the low-coverage limit.