The novel perovskites Sr1Co1-xMxO3-delta (M= V,Nb,Ti) have been evaluated as superior cathodes for solid-oxide fuel cells (SOFC); tests in single fuel cells yield output powers higher than 800 mW/cm2 at 850 C with H2 as a fuel. In this experiment we aim to unravel the oxide-ion diffusion path of this family of MIEC (mixed ionic-electronic conductor) oxides. We plan to study the thermal evolution of the crystal structure in order to get information about the actual crystal symmetry, the thermal vibrations and oxygen contents as well as the order-disorder of the oxygen vacancies and their evolution at the actual working conditions in a SOFC cell, in air at temperatures between 600 and 900 C.