To better understand thermal strain in electrochemical ceramics, the temperature and oxidation-state dependence of lattice volume in La0.6Sr0.4Co0.2Fe0.8O3-delta, (LSCF) were measured. Large values in the apparent thermal expansion coefficient at high temperature (> 50 ppm/degreesC) were caused by changes in oxygen content, not increases in thermal expansivity. This material can be described using an improved thermodynamic formalism that incorporates a new physical property, the chemical expansivity. Our approach opens new avenues for modeling stress and strain in materials, probing defect structure, and analyzing transport and kinetic properties.