Mixed [(1-x)BiO1.5-x MoO3] oxides in the composition range x=0 to 0.25 have been prepared by solid state reaction at different temperatures and their electric transport properties have been characterised as a function of temperature and oxygen partial pressure P-O2. Broad-range solid solutions belonging to the fluorite-related structure of delta-Bi2O3 are formed at high temperatures, and their high conductivity can be partially retained during fast cooling, but it is not reproducible during successive heating runs. Reproducible electrical properties have been obtained as a result of long thermal annealing at 800 degrees C. Materials so prepared show a conductivity maximum of similar to 10(-3) or 10(-2) Ohm(-1) cm(-1) (respectively at 500 or 700 degrees C) for compositions near x=0.125. For x=0.125 (Bi/Mo=7) the total bulk conductivity is independent of oxygen partial pressure in the range 1 less than or equal to P-O2 less than or equal to 10(-13) atm. Below the latter pressure, an irreversible conductivity jump occurs, due to formation of a reaction layer on the electrode surface.