We have carefully measured the thermal expansion for ceria nanoparticles across a wide range of crystallite sizes. Such an experiment probing the crystallite size dependency of thermal expansion has virtually no precedence. We found that as crystallite size decreases, the coefficient of thermal expansion decreases as well. We are able to calculate from the expansions the relative contributions of surface adsorbents and surface non-stoichiometry to surface stress. We find that smaller nanoparticles owe more of their surface stress to surface non-stoichiometry. (C) 2017 Published by Elsevier B.V.