Photon driven surface reactions of PH3 on GaAs(100) have been studied by x-ray photoelectron spectroscopy, temperature programmed desorption, and high resolution electron energy loss spectroscopy. Nonthermal excitation was achieved with an excimer laser producing 193 nm (ArF) and 248 nm (KrF) photons. Photons induce both desorption and dissociation of PH3 at 100 K. Phosphine coverage decreased monotonically with increasing photon fluence. Two dihydrogen desorption peaks at 500 and 600 K, found after irradiation with photons, were attributed to the desorption of hydrogen from Ga and P sites. The appearance of Ga-H stretches in the high resolution electron energy loss spectra following irradiation confirms that H is transferred from P to Ga during the irradiation process. The cross sections of the overall PH3 loss were 1.9 X 10(-18) and 2.9 X 10(-20) cm2 for 193 and 248 nm photons, respectively. X-ray photoelectron spectra of P(2p) show that some gallium phosphide was formed after photon irradiation. The observed reactions were attributed to both direct photon absorption by the adsorbate and to substrate mediated hot electron attachment processes.