The syntheses of the three forms of bismuth phosphate have been optimized, and the resulting single phases have been characterized by thermal analysis, infrared spectroscopy, and powder X-ray diffraction. Two phases may be obtained at room temperature, one trigonal (slightly hydrated) and another monoclinic with a monazite-type structure. The crystal structure of the trigonal phase, (stoichiometry BiPO4.0.67H2O, space group P3(1)21; a = 6.9885(2) angstrom, c = 6.4867(2) angstrom, Z = 3; R(WP) = 5.6%, R(F) = 3.6%) has been refined by the Rietveld method. The structure contains channels that run parallel to the c axis where the water molecules are located. This water is coordinated directly to the bismuth atoms and stabilizes this form respect to the monazite-type BiPO4 under the synthetic conditions. But the trigonal phase is metastable (at room temperature it spontaneously loses all the crystallization water in 2 months) and is transformed irreversibly to the monazite-type structure. The crystal structure of this second form (stoichiometry BiPO4, space group P2(1)/n; a = 6.7626(1) angstrom, b = 6.9516(1) angstrom, c = 6.4822(1) angstrom, beta = 103.74(1)degrees) has been refined to R(WP) = 6.6% and R(F) = 3.7%. The monazite-type BiPO4 slowly transforms into the high-temperature monoclinic form when it is heated above 600-degrees-C. The crystal structures of these three phases present some important similarities which are described in detail.