Phosphorus desorption following phosphine (PH3) adsorption on the Si(100) surface was analyzed via temperature-programmed de sorption (TPD) and Auger electron spectroscopy. Phosphine partially decomposes upon adsorption at room temperature. Hydrogen (H-2) and phosphine are produced via recombinative desorption during TPD. At higher temperatures the remaining phosphorus desorbs in the form of P-2. At low P coverages, a single alpha state is present in the desorption spectra which obeys second-order kinetics with an activation energy of 72 kcal/mol. At coverages around 21% of a phosphorus monolayer, a second beta state appears at lower temperatures which also obeys second-order kinetics with a desorption energy of 57 kcal/mol. At coverages greater than about 42% of a phosphorus monolayer, a third desorption state is observed.