It has been established that M(H2PO4)(2)center dot 2H(2)O (M = Mn, Co, Ni) are paramagnetics between 173 and 353 K with weak antiferromagnetic exchange interaction between the metal ions. In situ magnetic measurements during the thermal decomposition of the salts show that the oxidation state and the octahedral coordination of M2+ are preserved. From the data obtained it could be supposed that in M(H2PO4)(2)center dot 2H(2)O (M = Co, Ni) this process is topotactic with no long-range diffusion transport. In Mn(H2PO4)(2)center dot 2H(2)O, the formation of the large variety of intermediate products probably requires more drastic rearrangement and diffusion of the manganese ions during the complex transformations, which reflect on both the value and the sign of the theta constants. M2P4O12 (M = Mn, Co, Ni), which are the final decomposition products of the corresponding dihydrogen phosphates are paramagnetics in the temperature range of 295-573 K with antiferomagnetic interactions between the metal ions. The lattice parameters of Ni(H2PO4)(2)center dot 2H(2)O have been calculated. It crystallizes in the monoclinic system with a = 7.228(1) angstrom; b = 9.778(1) angstrom; c = 5.306(l) angstrom; = 94.50(1)degrees, SG P2(1)/n with Z = 2. (c) 2005 Elsevier Ltd. All rights reserved.