Two different nonstoichiometric mixed perovskites : Sr2(Sc1+Nb1-x)O6-delta (with x=0.05 and 0.1) and Ba3(Ca1.18Nb1.82)O9-delta, are shown to become good high-temperature protonic conductors upon exposure to H2O (or D2O) atmospheres. Conductivity is studied in the temperature range of 300 approximately 550 K where the proton concentration is frozen in. The activation energies E(H) for proton migration in these compounds are 0.62 and 0.54 eV, respectively. The conductivities fall in the same range as those for M3+-doped SrCeO3 and BaCeO3, but the present materials do not show electronic conduction after treatment in highly reducing atmospheres as do the cerates. It is therefore concluded that these compounds deserve serious consideration as fast high-temperature protonic conductors.