High-resolution x-ray photoemission spectroscopy and Ar-ion bombardment have been used to study temperature-dependent chemical reaction and species redistribution for Ti/GaAs(100). Our results show that Ti deposited at room temperature disrupts the GaAs substrate by reacting with As and releases Ga into the overlayer. As is found to accumulate near the buried interface in the form of a Ti-As compound. Ga is depleted from, but accumulates beyond, this reacted region. Sputter-depth profiles indicate that high-temperature annealing causes Ti diffusion into the GaAs substrate and enhanced reaction with As. Ga expulsion from the forming Ti-As compound becomes more severe when the amount of Ti-As increases. Heating promotes segregation of ejected Ga atoms to the vacuum surface, but has little influence on As segregation.