La(PO3)3aCa(PO3)2 meta-phosphate glassaceramic composites were prepared by quenching from the melt, followed by heating at 800 degC for 20 h, in air. Microstructural characterization showed that exposure to water vapor pressures equal to or in excess of 4.2 kPa at as low as 550 degC resulted in the disproportionation of the lanthanum metaphosphates, with formation of some LaPO4. This reaction puts important limitations on the conditions under which the lanthanum metaphosphates could be used as electrolytes in electrochemical devices. The conductivities at uniform air/water partial pressures around 3 kPa, were independent of the water partial pressures used. Comparison of the total ionic effective transference numbers and the proton effective transference number suggested that electronic transport contributed on the average about 7% to the total conductivity in the examined temperature range. The measurements suggested that oxygen ions also participate. The H/D isotope effect, assessed from the ratio IH2O/ID2O ranged from 1.28 at 400 degC to 1.35 at 500 degC, evidencing dominant proton conduction. At 550 degC, the proton effective transference numbers, tH+, determined from the OCVs of cells with oxidizing differential atmospheres were around 0.87, in good agreement with the value derived from the IH2O/ID2O conductivity ratio at 550 degC. However, at lower temperatures such agreement was not found.