The adsorption and decomposition of PH3on Si(111)‐(7×7) was investigated in ultrahigh vacuum by means of temperature programmed desorption, low‐energy electron diffraction, Auger electron spectroscopy (AES), and electron stimulated desorption (ESD) methods. Phosphine adsorbs on Si(111)‐(7×7) atT=120 K with an initial sticking coefficient ofS0&bartil;1 through a mobile (extrinsic) precursor state. Some PH3dissociative adsorption at 120 K is observed. Thermal activation of the adsorbed species results in desorption of a molecular PH3species up to 550 K. Further heating produces H2(g) desorption atT&bartil;740 K and P2(g) desorption atT&bartil;1010 K, thus indicating that PH3decomposition has occurred. AES and ESD studies of the adsorbed species reveal that decomposition takes place by the breaking of PH bonds in PHx(a) to form SiH species on the surface for 120 K<T<700 K.