LaPO4 with nominally 5 atom% Ca or Sr was synthesised using co-precipitation, cold-pressed to bars, and sintered at 1300 degrees C. The electrical conductivity in wet air was mainly protonic up to approximately 900 degrees C; it increased with increasing humidity and decreased when protons were replaced by deuterons in situ by equilibrating the samples in air containing D2O. Above 900 degrees C the proton conductivity fell and a native conductivity contribution became dominant.