The adsorption and decomposition of PH 3 and its coadsorption with CO and deuterium were investigated by TPD and HREELS on a Pt(111) surface. At 100 K, phosphine adsorbs associatively via a precursor state with a saturation coverage of 0.5 ML. HREELS results are consistent with C 3v symmetry for PH 3 and bonding through the P lone pair. Vibrational spectra indicate little perturbation of the P-H bonds upon adsorption. Upon heating, P-H bond breaking occurs as low as 160 K and is completed by < 300 K. No PH 3 desorbs intact. The AES P signal shows both reversible and irreversible characteristics as a function of temperature. Diffusion into and out of the bulk and compound formation are suggested. Preadsorbed deuterium has little effect on adsorption of PH 3, but PH 3(a) blocks D 2 adsorption. Slow exchange of D for H into PH 3 occurs at 150 K, but not at 100 K. Phosphine blocks sites for CO adsorption and lowers the sticking coefficient at 100 K for the latter so that at saturation PH 3(a) no CO will adsorb. On the other hand, PH 3 will readily adsorb on and will displace up to 0.14 ML of a nearly saturated layer of CO. Coadsorbed PH 3 causes CO to shift from atop to bridge sites. Carbon monoxide forms a weak “H-bond” with coadsorbed PH 3.