An isothermal section of the Al–Li–O system has been constructed at 1000 K using available thermodynamic data. Oxide stability is shown as a function of alloy composition and oxygen partial pressurepO2). Rigorous experimental verification of the calculated phase relations is precluded by the extremely low values ofpO2) at equilibrium. The validity of the computed phase relations is considered from the results of laboratory studies of refractory compatibility and observations of interfacial phases in aluminum alloy matrix composi