La1-xSxPO4 samples of nominal composition x=0.025 and x=0.050 were prepared by powder combustion synthesis, cold-pressing, and sintering at 1300degreesC. Samples with Pt paste electrodes were characterized by conductivity measurements, impedance spectroscopy, and transport number measurements. The samples are predominantly protonic conductors below 700degreesC in wet oxygen, as proven for the first time with the EMF/water vapour concentration cell method. The modelling of the conductivity data over a large temperature range suggests that the entropy and enthalpy changes of the incorporation of water vapour to hydrolyse the P2O74- group into two HPO42- groups are -90 J/mol K and -83 kJ/mol, respectively. Impedance spectroscopy showed that gain boundary resistance is dominant only below 400degreesC and thus not a severe problem for applications. The synthesis procedure is convenient and upscaleable, but at this stage, the proton conductivity appears to be somewhat lower than reported elsewhere, assigned in part to insufficient dopant homogenisation. (C) 2003 Elsevier B.V. All rights reserved.