Adsorption of phosphine on indium phosphide compound semiconductor surfaces is a key process during the chemical vapor deposition of this material. Recent experimental infrared studies of the In-rich InP surfaces exposed to phosphine show a complex vibrational pattern in the P-H stretch region, presumably due to overlapping contributions from several structural species. We have performed density functional calculations using finite-sized cluster models to investigate the dissociative adsorption of PH3 on the In-rich InP surface. We find that initially PH3 forms a dative bond with one of the surface In atoms with a bindin,g energy of approximately 11 kcal mol(-1) at 298 K. The In-PH3 bond length is 2.9 A, 0.3 A greater than the In-P covalent bond length computed for In-PH2 species produced by hydrogen migration to a neighboring atom. However, the dissociation process, though exothermic, involves a significant activation barrier of similar to 23 kcal mol(-1), suggesting the possibility of metastable trapping of the dative bonded PH3 molecules. Indeed, a careful vibrational analysis of different P-H stretching modes of the surface-bound PH3 and PH2 units gives excellent agreement with the observed infrared frequencies and their relative intensities. Moreover, at higher temperatures the frequency modes associated with PH3 disappear either due to desorption or dissociation of this molecule, an observation also well supported from the computed thermochemical parameters at different temperatures. The computed energy parameters and infrared analysis provide direct evidence that PH3 is present as a dative bonded complex on the InP surface at room temperature.