The adsorption and thermal decomposition of phosphine (PH3) on the Si(001) surface has been investigated using Fourier transform infrared spectroscopy. Phosphine is found to adsorb both nondissociatively and dissociatively, depending on the coverage and flux during exposure. Infrared spectra reveal that PH3 dissociates to produce two distinct forms of PH2. Further decomposition yields phosphorus atoms and surface hydrogen. The surface P atoms form P-Si heterodimers and, in the presence of surface hydrogen, form PSiH "hydrided heterodimers". The bonding of hydrogen in this form is stronger than on the clean Si surface, resulting in a higher desorption temperature for hydrogen when phosphorus is present on the surface. Molecular orbital calculations are used to provide insight into the bonding geometry of PH3 on Si(001).