An expanded use of the mass-loss Knudsen effusion experiment is described. The nonstoichiometry created by ionic substitutions in complex oxide systems that contain only one volatile oxide component, can be determined at temperature. A two-phase sample of known composition, prepared rich in the volatile component, is sealed within a Knudsen cell and heated isothermally in vacuum. The continuous effusion of the volatile oxide introduces linear mass-loss, and gradually depletes the second phase, whereupon mass-loss becomes non-linear. The amount of linear mass-loss establishes the concentration of the volatile oxide component within the single-phase sample and allows description of the sample's defect structure. The method is illustrated at 1100°C by resolving the nonstoichiometry produced by the equilibrium substitution of 8 mole % La3+ into Pb(Zr0.65Ti0.35)O3, which vaporizes only PbO. The experiment eliminates several defect models, and demonstrates that La3+ substitutes completely on the Pb2+sublattice, with charge neutrality requirements satisfied by forming both (Pb2+) and (Zr4+, Ti4+) sublattice vacancies.