We used infrared absorption spectroscopy in the multiple internal reflection geometry to investigate the adsorption of SiH 4 on the Si(001)(2x1) surface. Comparing infrared data with the density functional cluster calculation, we show that at low hydrogen coverage the silane molecule dissociatively adsorbs on Si(001)(2x1) to populate a dihydride (SiH 2 ) at the bridge site between two adjacent dimers and monohydride species; monohydrides are formed by terminating the unsaturated dangling bonds of dimers by hydrogen atoms released from the silane molecule. We suggest that at high hydrogen coverage, silane adsorbs onto a single dimer to generate monohydride and sylil groups (-SiH 3 ). We also demonstrate that the dihydride species that is initially generated by silane adsorption, decomposes to monohydride species even at room temperature.