Cubic sillenite-type and monoclinic bismuth phosphates were synthesized via hydrothermal methods. Structure and morphology of cubic Bi13.1POδ and monoclinic BiPO4 were characterized with a variety of analytical methods including powder X-ray diffraction, electron microscopy methods and surface area measurements. The humidity sensing properties of both bismuth phosphate types were investigated with respect to their capacitance characteristics. Sillenite-type cubic bismuth phosphate exhibits better response behavior than monoclinic BiPO4. Cubic bismuth phosphate displays a capacitance change of up to 4 orders of magnitude over a relative humidity (RH) range from 11% to 95% together with a linear adsorption/desorption relationship. This promising humidity sensing behavior is related to the presence of a framework structure containing polarizable Bi3+ cations in combination with moderate phosphate doping.