Cobalt-doped praseodimium nickelates PrNi1 - xCoxO3 - δ and Y/(Pr, Y)-doped ceria oxides were synthesized via Pechini route. Nanocomposites were prepared via ultrasonic dispersion of the mixture of perovskite and fluorite nanopowders in isopropanol with addition of polyvinyl butyral followed by drying, pressing and sintering at 1000-1100°C. The oxygen mobility and reactivity of perovskites, fluorites and powdered composites obtained by crushing and milling of pellets were estimated by oxygen isotope heteroexchange with 18O2 and C18O2 using both closed and flow (SSITKA) reactors in the temperature-programmed (TPIE) mode. Co-existence of two routes of oxygen bulk diffusion-the fast one and the slow one in perovskites and composites was revealed. In perovskites, the fast oxygen diffusion is aminor channel related to some defects in their structure. In composites, the fast oxygen diffusion is the main route being related to both phases disordered due to cation redistribution between them, first of all, to fluorite-like domains of Pr,Y-doped ceria and perovskite/fluorite interfaces.