A complete series of solid solutions was prepared in the SrZr(PO 4 ) 2 -BaZr(PO 4 ) 2 system and examined by conventional X-ray powder diffraction (XRPD). The crystals of Sr x Ba 1Àx Zr(PO 4 ) 2 with xp0.1 were isomorphous with yavapaiite (KFe(SO 4 ) 2 , space group C2/m). The solid solution with 0.2pxp0.7 has been composed of a new phase, showing a superstructure along the a-axis (c-axis of the yavapaiite substructure). The crystals with 0.8pxp0.9 were composed of both the new phase and the triclinic phase, the latter being isostructural with SrZr(PO 4 ) 2 (x ¼ 1). The crystal structure of the new phase has been determined using direct methods, and it has been further refined by the Rietveld method. The crystal of Sr 0.7 Ba 0.3 Zr(PO 4 ) 2 (x ¼ 0.7) is monoclinic (space group P2/c, Z ¼ 4 and D x / Mg m À3 ¼ 3.73) with a ¼ 1.53370(8) nm, b ¼ 0.52991(3) nm, c ¼ 0.84132(4) nm, b ¼ 92.278(1)1 and V ¼ 0.68321(6) nm 3 . Final reliability indices are R wp ¼ 7.32%, R p ¼ 5.60% and R B ¼ 3.22%. The powder specimen was also examined by high-temperature XRPD and differential thermal analysis (DTA) to reveal the occurrence of two phase transitions during heating; the space group changed from P2/c to C2/m at $400 K, followed by the monoclinic-to-hexagonal (or trigonal) transition at 1060 K. The P2/c-to-C2/m transition has been, for the first time, described in the yavapaiite-type compounds.