Two new cobalt zinc orthophosphate hydrates with similar chemical formula, (CoxZn(1-x))3(PO4)2.H2O, but different composition and structure, have been prepared by systematic hydrothermal synthesis from the system nCo(CH3COO)2 : (1 -n)Zn(CH(3)COO)2 : 3.5H3PO4 : 2.1(CH3)2NH(CH2)3NH2:144H2O (0 </= n </= 1). The material Co(2.59)Zn(0.41)(PO4)2.H2O 1 has a three-dimensional structure that can be considered to be built from layers of edge-sharing CoO(6) octahedra joined by edge-sharing (Co/Zn)O(5) trigonal bipyramids, which also share edges with PO(4) tetrahedra. Compound 2, Co(0.72)Zn(2.28)(PO(4))(2).H(2)O, is isostructural with a known phase of Zn(3)(PO(4))(2).H(2)O: its structure contains corner-sharing (Zn/Co)O6 octahedra, (Zn/Co)O4 tetrahedra and PO4 tetrahedra, forming channels into which the coordinated water molecules project. Magnetic susceptibility measurements for 1 and 2 are consistent with the chemical compositions determined by the single-crystal X-ray analyses and with the presence of Co2+. The range for possible Co/Zn substitution in 1 and 2(assessed by EDX analysis) is relatively small: x lies in the range 0.74-0.80 (+/- 0.05) for 1 and 0.23-0.28 (+/- 0.05) for 2. Thermal investigation of 1 and 2 by thermogravimetry (TG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) shows that both materials transform to gamma-(CoxZn(1-x))3(PO4)2 when heated to 518 and 435 degrees C, respectively, with enthalpy changes for complete dehydration of DeltaH= 41.9 and 53.5 kJ mol(-1), respectively. Dehydration of 1 occurs in a single irreversible step, while that of 2 occurs over a greater temperature range and proceeds via several steps. A new phase, (CoxZn(1-x))3(PO4)2.0.27H2O, is formed when 2 is heated to 357 degrees C.