To understand the mechanism of oxygen diffusion, the oxygen tracer diffusion coefficient, D-0*, of strontium-doped lanthanum manganites, La1-xSrxMnO3+/-delta (x = 0.05, 0.10, 0.15 and 0.20) was measured as a function of composition, temperature and oxygen partial pressure. The measured D-0* was as low as 10(-12) to 10(-11) cm(2) s(-1) at 1000 degrees C, from which the ionic conductivity of 10(-7) to 10(-6) S/cm was estimated. Because of such a poor ionic conductivity, the possibility for the bulk diffusion of oxide ions to contribute to the cathode reactions in solid oxide fuel cells is considered to be sparse. The negative dependence of D-0* on the oxygen partial pressure suggests the vacancy mechanism for oxygen diffusion.