MgO has important potential uses as a buffer layer for growth of perovskite and related oxides on GaAs. Equilibrium thermochemical data indicate MgO will be stable against interfacial reaction with GaAs but, under molecular beam epitaxy growth conditions, kinetic limitations can increase the probability of an undesired Mg or O reaction with the substrate and influence epitaxial growth. III situ reflection high-energy electron diffraction and Auger spectroscopy were combined with ex situ atomic farce microscopy to study the potential for nonequilibrium processes to affect MgO growth on GaAs (001) by reactive molecular beam epitaxy. Under oxygen poor conditions, Mg reacts with GaAs to form an epitaxial phase with a cubic structure and a lattice constant of 0.62 +/-0.01 nm. The growth mode is highly three-dimensional, producing a rough surface morphology. This phase is unstable to subsequent oxidation, which leads to segregation of Mg to the surface to form a polycrystalline MgO overlayer. Three-dimensional growth of the reaction product is inhibited by oxygen pressures providing a Mg-to-O-2 flux ratio on the order of 2, for growth temperatures at 700 K. However, the epitaxial-quality of MgO is still degraded at these oxygen pressures suggesting that, close to stoichiometric MgO conditions, the Mg-GaAs reaction still influences the growth of epitaxial MgO. [S0734-2101(98)06504-X].