La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) displays mixed ionic electronic conductivity at high temperature that makes it suitable for solid oxide fuel cell cathodes, ceramic oxygen generators, and oxygen permeation membranes. However, upon changes in temperature and oxygen partial pressure LSCF exhibits changes in volume, termed chemical expansion, which can result in mechanical failure of the aforementioned devices. The chemical expansion change with temperature also complicates thermal expansion modeling. The present work measures and models chemical expansion as a function of oxygen partial pressure (10−4−1 atm O2) and temperature (700°–900°C) using our previously modeled defect equilibria to determine the chemical expansion coefficient per oxygen nonstoichiometry (δ) of 0.031. The thermal expansion is also measured and modeled using the Morse potential and Debye frequency distribution. The results are combined to yield a model describing the thermo-chemical expansion of LSCF.