The dissociative chemisorption of phosphine, PH 3 , on the Si(100) surface has been examined with supersonic molecular beam scattering techniques. The effect of phosphorus adatoms, P(a), formed by the dissociative chemisorption of PH 3 , on the reaction of SiH 4 and Si 2 H 6 with Si(100) has also been characterized. All reactions have been examined at substrate temperatures characteristic of steady-state Si thin film growth, i.e. T s ≥450°C. The reaction probability, S R for PH 3 on Si(100) decreases with increasing substrate temperature and the kinetic energy of the incident molecule, suggesting that chemisorption proceeds via a trapping precursor-mediated dissociation channel. The dependence of PH 3 dissociative chemisorption on the fractional coverage of P(a) and H(a) has been deduced under conditions where the desorption of H 2 and P 2 are finite. The dependence of the dissociative chemisorption of both SiH 4 and Si 2 H 6 on the coverage of P(a) has also been determined. For reaction conditions under which P(a) is the dominant surface species, the reaction probabilities of PH 3 , SiH 4 and Si 2 H 6 are proportional to the quantity 1-θ P 2 , where θ P is the fractional coverage of the adsorbed phosphorus atoms. In addition, the presence of phosphorus adatoms has also been found to have a significant influence on the reaction pathway of Si 2 H 6 on Si(100). Our results have been employed to formulate a predictive model for the kinetics of Si thin film growth in presence of PH 3 (g).