Photoemission experiments have shown that strong chemical reactions occur at clean Pd/GaAs(110) and Ti/GaAs(110) interfaces formed at room temperature in ultrahigh vacuum. In both cases elemental Ga was found dispersed in the metal film, which also had a metal-As compound segregated to its surface. In addition, what appears to be elemental As was observed on the Pd-covered surfaces. Subsurface reactions were identified well below monolayer (ML) coverages. The evolution of band bending with increasing metal coverage starting near 0.001 ML is strongly dependent on the metal and results in Schottky-barrier heights of 0.73 (0.70)±0.05 eV for Ti and 1.01 (0.42)±0.05 eV for Pd on n-type (p-type) GaAs(110). Strong emission from the transition-metal d orbitals in the band-gap region of the GaAs was observed for coverages 0.01 ML and represents the first direct spectroscopic identification of occupied interface states responsible for Fermi-level pinning. The latter observation together with differing coverage dependences for the final pinning positions for Pd and Ti indicate a new mechanism for the formation of a Schottky barrier. It is suggested that rebonding between the transition metal and the GaAs at the interface produces partially occupied d orbitals which lie in the semiconductor band gap. In addition, several refinements of the photoemission technique and data analysis are discussed and shown to be relevant to both the accurate determination of band bending and the chemical characterization of the interface. © 1986 The American Physical Society.