Many nickel-base superalloys exhibit creep embrittlement after exposure to oxygen-containing environments at high temperatures. This embrittlement is characterized by a region of profuse grain boundary cavitation immediately below the surface, and four mechanisms have been proposed in the literature to account for it. Since each mechanism has microstructural consequences, these have been checked metallographically. From the results given in this paper it is concluded that, for two nickel-base superalloys, cavitation is nucleated by bubbles of a gaseous internal oxide of carbon.