New binary metal dihydrogen phosphate dihydrate Mn0.5Cu0.5(H2PO4)(2)center dot 1.5H(2)O was synthesized by a rapid and simple coprecipitation method using phosphoric acid, manganese metal, and copper oxide at ambient temperature. The thermal stability of Mn0.5Cu0.5(H2PO4)(2)center dot 1.5H(2)O was studied by means of the non-isothermal kinetics (Kissinger method). The synthesized Mn0.5Cu0.5(H2PO4)(2)center dot 1.5H(2)O shows complex thermal transformations, and its final decomposition product was a binary metal cyclotetraphosphate, MnCuP4O12. The X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis-near-IR, and Fourier transform IR (FTIR) results of the synthesized Mn0.5Cu0.5(H2PO4)(2)center dot 1.5H(2)O and the decomposed MnCuP4O12 appear to be very similar to those of M(H2PO4)(2)center dot 2H(2)O and M2P4O12 (M = Mn and Cu), which indicate the monoclinic phase with space group P2(1)/n and C2/c, respectively. The dominant features of the synthesized Mn0.5Cu0.5(H2PO4)(2)center dot 1.5H(2)O and the decomposition product MnCuP4O12 are compared with those of M(H2PO4)(2)center dot 2H(2)O and M2P4O12 (M = Cu and Mn), respectively.