The results of oxygen nonstoichiometry are presented for the chromite LaCr0.79Cu0.05Al0.16O3-delta with hexagonal perovskite structure at 1000 degreesC. The modeling of the defect structure of this oxygen deficient chromite is carried out by considering also the acceptor dopant Cu on B-site. The equilibrium constants, necessary to calculate all a priori unknown equilibrium defect concentrations, are determined by fitting the derived model equation to the measured oxygen nonstoichiometry. The three oxidation states 3+, 2+, and 1+ for copper are considered simultaneously in Cu-doped lanthanum chromite in addition to 3+ and 4+ states for chromium in the host lattice. The results of the fitting revealed that there is no significant change of the Cr3+ oxidation state over the complete oxygen partial pressure range investigated (-20less than or equal tolog(p(O2)/atm)less than or equal to0). In contrast to this, copper changes its oxidation state continuously from 3+ via 2+ to 1+ if the oxygen partial pressure decreases thereby giving rise to oxygen vacancy formation. (C) 2004 Elsevier B.V. All rights reserved.