The dissociative adsorption of phosphine (PH3) on Si(100)-(2×1) and its high temperature thermal behavior have been studied by high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy, and by temperature programmed desorption (TPD). Phosphine adsorbs dissociatively onto Si(100)-(2×1) at 100 K as PH2 and H species, as revealed by vibrational bands at 1050 cm−1 [δsc(PH2)] and 2100 cm−1 [ν(Si–H)]. The PH2(a) undergoes thermal decomposition to adsorbed P and H near 650 K, as determined by HREELS. TPD measurements reveal two PH3 desorption processes at 485 and 635 K. The 635 K desorption is shown to result from PH2+H recombination, while the mechanism for the 485 K desorption cannot be definitely identified. Additionally, two H2 desorption states were observed at 685 and 770 K.