Measurements of the thermoelectric power were applied to investigate the electrical conductivity in 5 mol % Sr-substituted LaPO4 as a function of temperature (600 to 900°C), water vapor partial pressure (0.2 to 6 kPa), and oxygen partial pressure (4 to 100 kPa). Expressions of the thermoelectric power for materials conducting protons, oxide ions, and electronic defects were derived, based on a thermodynamic treatment of entropy production by heat and charge transfer. The experimental data for 5 mol % Sr-substituted LaPO4 were interpreted in terms of protonic conduction, and some additional conduction, possibly by electron holes. In air, the proton transport number is unity at 600°C when PH2O > 1 kPa, and at 700°C when PH2O > 3 kPa. In wet atmospheres, the transported entropy of protons in this material was found to be nearly constant, 112 ± 2 J mol-1 K-1 at 600 to 800°C in wet atmospheres.