The adsorption of H2S on to the Si(001)-(1 x 2) surface is studied, based upon ab-initio pseudopotential calculations. We have used a dissociative adsorption model with adsorbed species (SH)(-) and H+. The sulphur atom is located along the dangling bond direction on one component of the Si dimer, the I-I atom in the (SH)(-) complex is bonded to the S atom, and H+ is bonded along the dangling bond direction of the other dimer component. The adsorption of the molecule removes the buckling of the Si dimer. Our calculated bond lengths for Si-S, Si-H and S-H are 2.15, 1.53 and 1.41 Angstrom, which are very close to the sum of their corresponding covalent radii. We find that the adsorption of H2S passivates the Si(001) surface. The system is characterized by the presence of a single surface state just below the bulk valence band maximum. (C) 1999 Elsevier Science B.V. All rights reserved.