Local interface chemistry is important to understand before proceeding to unravel the electronic properties of interfaces. Characterization of the intrinsic aspects of reactions and interdiffusion is also important to understand possible device size limitations. Various types of local reactions are possible and we have investigated these with surface sensitive photoemission and photoyield spectroscopy using synchrotron radiation excitation. We show for both compound semiconductor and for oxide surfaces that reactions can occur which locally determine the chemical phases present. The heat of these reactions can drive the interdiffusion process. We present results for the important systems Al–GaAs, Al-oxidized GaAs and Al–SiO2. We compare these results with our previous results for Ge and Ga on GaAs as well as those obtained by others. We demonstrate that the I–V behavior of Ga on p-GaAs is ohmic and attribute this non-schottky character to As diffusion and vacancy production at the interface. Diffusion can clearly be identified as distinct from island formation by analyzing the core photoelectron line shape; Ge grown on GaAs(110) at differing substrate temperatures provides an illustrative example. Interfacial reactions are observed in real time for Al2O3 formation by Al reduction of oxidized Si.