This paper is the first part of a two part series, where the effects of varying the A-site dopant on the defect chemistry, the diffusion coefficient and the surface catalytic properties of the materials (La0.6Sr0.4 − xMx)0.99Co0.2Fe0.8O3 − δ, M = Sr, Ca (x = 0.05, 0.1), Ba (x = 0.1, 0.2) (LSMFC) have been investigated. In part I, the findings on the defect chemistry are reported, while the transport properties are reported in part II. Substitution of Sr2+ ions with Ca2+ ions (smaller ionic radius) and Ba2+ ions (larger ionic radius) strains the crystal structure differently for each composition while keeping the average valence of the cations constant. The Ba2+ containing materials show the largest oxygen loss at elevated temperatures, while the purely Sr2+ doped material showed the smallest oxygen loss. This was reflected in the partial oxidation entropy of the materials. The measured oxygen loss was modelled with point defect chemistry models. Measurements at very low pO2 showed several phase transitions.