Theoretical calculations using a modified version of CNDO/UHF method to correct binding energies were carried out on a series of linear MoxSy (x = 3; y = 6, 8, 10, 12, 14 and x = 5; y = 12, 14) and non-linear (x = 3, y = 6, 8, and x = 5, y = 6, 8, 10, 12, 14, 16, 18, 20) clusters. Results show the formation of S-2(-) species on the surface and the existence of a Mo-Mo interaction. The HOMO orbitals are localized on mono-coordinate S atoms which shows that these atoms have electron-donating properties while the LUMO orbital is delocalized over all coordinatively unsaturated Mo atoms which presents electron-acceptor properties. It is shown that the non-linear clusters are better models for representation of MoS2 catalysts, because they of their stability respect to the linear clusters. (C) 2002 Elsevier Science B.V. All rights reserved.