E.s.r.-detectable superoxide ions are formed at the surface of MgO outgassed at high temperature (1073 K) by the coadsorption of dioxygen and slightly acidic X—H molecules like benzene, ethylene, ammonia, toluene, propene, acetylene and hydrogen. The basic step in O–2 formation is electron transfer from the X-anion formed upon dissociative adsorption of X—H. The superoxide ion acts as a probe of the electric field at the surface site. Two types of spectra have been observed, both of which have been computer-simulated. Type I has four components, is characteristic of mild X—H/O2 interactions, and monitors the presence of four different coordination states of Mg2+ cations on unperturbed MgO microcrystals. The difference gzz–ge correlates well with the Madelung constants at four definite locations at the surface, which are proposed as adsorption sites. Type II spectra, which have one–two components, have been obtained under more severe conditions, and are ascribed to sites modified by the presence of products arising from oxidation reactions of the X—H molecule. The Känzig and Cohen model shows some weakness as far as the prediction of gxx components is concerned.