Oxygen stoichiometry and transport in (La1-xSrx)(0.95)FeyMn1-yOz (0 less than or equal to x less than or equal to 0.30, 0 less than or equal to y less than or equal to 0.7) was investigated by thermogravimetry at 800-1000 degrees C upon repetitive changes between N-2- and N-2/O-2-atmospheres and by chemical analysis. The materials was found to be overstoichiometric with respect to oxygen. in air at 1000 degrees C at low substitution levels changing to understoichiometric at the highest substitution levels. The oxygen transport kinetics and the oxygen diffusion coefficients were determined from the weight changes vs. time and from SEM and X-ray data on crystallite dimensions. For powdered samples, oxygen diffusion coefficients in the range 10(-15)-10(-11) cm(2)/s are reported.