Nonstoichiometry of the perovskite-type solid solutions La 0.9 Ca 0.1 Cr 1−y Al y O 3−δ was studied by high-temperature gravimetry under controlled P(O 2 ) atmospheres of 1–10 −23 bar at 1073–1273 K. The observed data were described by a regular solution-like model for the randomly distributed defects of V O ¨, Cr Cr ., Ca La ,, and Al Cr X . With the increase in y, V O ¨ formation becomes much easier. For y>0.8, some fraction of Ca La , becomes surrounded only by Al Cr X and V O ¨ remains around such Ca La , up to high P(O 2 ) to reduce the maximum oxygen content below 3.000.