The solid copper(I) electrolytes: CuTi2(PO4)(3); CuTiZr(PO4)(3); and CuZr2(PO4)(3); were prepared as powders by high temperature synthesis and analysed by powder XRD. These materials were then annealed in air at 400 A degrees C for 72 h. The results of powder XRD showed that the degree of oxidation under these conditions varies progressively and enormously across this series, with the passivity dependent upon the Ti/Zr ratio; CuTi2(PO4)(3) being the least reactive under these conditions. The results of the thermogravimetric analyses in artificial air (PO2 = 0.2 bar) corroborate with the above, and reveal in all cases that T (eqm) = 500 +/- A 25 A degrees C for the reversible reaction: 4Cu (Ti, Zr)(2)(PO4)(3) + O-2 a double dagger dagger 4Cu(0.5) (Ti, Zr)(2)(PO4)(3) + 2CuO. Green Cu0.5TiZr (PO4)(3) has been prepared as a new compound and was shown to belong to a rhombohedral system with hexagonal cell constants: a = 8.599(1) ; c = 22.355(3) ; Z = 6.