It is argued that compositional heterogeneity in certain reducible mixed oxides, like Ce1-xZrxO2, can be inferred from a variation in the metals ratio, as measured by x-ray photoelectron spectroscopy, between fully oxidized and partially reduced states of the oxide, provided that the characteristic linear dimension of the heterogeneity is at least as large as the typical electron inelastic mean-free path. The argument is supported by experimental results from two examples involving mixed phases, one in which the length scale is 100 nm and the other in which the domains are too small for detection by standard x-ray diffraction.