Electro-chemo-mechanical coupling involving the interplay between electrical, chemical, and mechanical states of materials, must be understood to improve materials performance (e.g., durability, efficiency) in solid-state electrochemical devices. Especially, Mixed ionic – electronic conducting oxides (MIECs) can undergo changes in stoichiometry with variations in the surrounding atmosphere, temperature, and voltages during process and operation. Such stoichiometry changes are also typically accompanied by the lattice distortions/strains, which is also referred to as chemical expansion, εc. Large chemical strains are typically associated with significant mechanical stresses, resulting in electrode cracks. Using Pr0.2Ce0.8O2−δ(PCO20) as a model system, our previous work found such deleterious chemical expansion can be successfully suppressed by reducing the particle size. In this presentation, we will present a detailed analysis of the coefficient of chemical expansion (CCEs) vs. particle size, with quantitative measurements of the oxygen sub-stoichiometry by thermogravimetric analysis (TGA). The CCEs, defined as chemical strain over change in stoichiometry, are an important property that can reveal if the reduced chemical expansion is solely due to limited changes in defect concentrations. Here, we found the smaller particles generally exhibit lower CCEs compared to the larger particles, possibly due to the greater likelihood of defect interactions/clustering at higher oxygen substoichiometries. Additionally, in contrast to the typical linear strain vs. stoichiometry change behavior (constant CCE) observed for PCO bulk, the nanopowders in this study exhibit a highly non-linear relationship (variable CCE), which could be ascribed to the different chemical/coordination environment of the formed oxygen vacancies when the oxygen sub-stoichiometry changes. The potential impacts of oxygen vacancy positions and surface vs. bulk differences in chemical composition/valence states on CCEs will be further discussed.