We present results of nb initio calculations, based on pseudopotentials and the density functional theory, for the dissociative adsorption of Si2H6 on the Si(001) surface. Various models are considered, containing H and the radicals SiH3 and SiH2. Models based on the SiH2 radical were considered with three adsorption sites: (i) an on-dimer position, (ii) an intrarow position between two neighboring Si dimers in the same dimer row, and (iii) an inter-row position between adjacent Si dimer rows. The intrarow and bridge geometries are considered with and without the saturation of the Si dangling bonds with hydrogen. For the 2 x 1 surface reconstruction, the on-dimer geometry is energetically more favorable than the inter-row geometry. For the 2 x 2 reconstruction, without hydrogen passivation of the Si dangling bonds, the on-dimer and intrarow geometries leave the system fully passivated. With hydrogen passivation of the Si dangling bonds, the on-dimer geometry is more favorable than the intrarow geometry.