Adsorption of OCS on GaAs(100) has been investigated using temperature-programmed desorption and photoinduced desorption/dissociation spectroscopy. It was observed that OCS molecularly adsorbs on GaAs(100) and thermally desorbs at 160 K. Adsorbed OCS molecules either dissociate into gaseous CO, leaving S adsorbed on GaAs(100), or desorb as molecular OCS under irradiation of lambda = 300 similar to 770 nm. The photodissociation cross section is about 5.0 x 10(-19) cm(2) at 300 nm, which is about 2 orders of magnitude higher than that for gaseous OCS at 250 nm and even 10 times greater compared with that for OCS on Ag(111) at 254 nm. It drops rapidly as the wavelength increases and becomes undetectable at about 480 nm. In addition, the photodesorption cross section was found to be about 8.4 x 10(-20) cm(2) at 300 nm with a threshold at about 770 nm. We also observed that the photodissociation cross section is strongly dependent on the coverage of OCS on GaAs(100), changing from 2.0 x 10(-18) cm(2) for 0.03 L OCS on GaAs(100) at 320 nm to 3.6 x 10(-19) cm(2) for 3.3 L OCS. A 2.4 eV red shift of threshold photon energy for photodissociation of OCS on GaAs(100) was observed in our experiment relative to gaseous OCS. Our results support the interaction of photogenerated carriers with adsorbed OCS in the photochemical processes. The observed difference in the threshold energies for photodissociation and photodesorption is possibly attributable to the different final states involved.