The superparamagnetic Mn1/2Fe1/2(H2PO4)(2)center dot 2H(2)O was synthesized by a soft synthesis method using a Mn(c)-Fe(c)-H3PO4 system in water acetone medium at ambient temperature. The synthesized Mn1/2Fe1/2(H2PO4)(2)center dot 2H(2)O decomposed through the dehydration and the phosphate condensation reactions at high temperature and yielded binary manganese iron cyclotetraphosphate MnFeP4O12. The XRD and FTIR results of the synthesized Mn1/2Fe1/2(H2PO4)(2)center dot 2H(2)O and the decomposed MnFeP4O12 indicate the pure monoclinic phase with space group P2(1)/n and C2/c, respectively. The thermal behaviors and superparamagnetic properties of Mn1/2Fe1/2(H2PO4)(2)center dot 2H(2)O and MnFeP4O12 in this work differed from the single compounds (M(H2PO4)(2)center dot 2H(2)O and M2P4O12, where M = Mn, Fe) and the binary compounds (Mn0.5Fe0.5(H2PO4)(2)center dot xH(2)O) reported in previous works. The kinetic and thermodynamic functions for thermal decomposition of Mn1/2Fe1/2(H2PO4)(2)center dot 2H(2)O were studied and confirmed as reaction mechanisms. Vibrational frequencies of breaking bonds in two thermal transformation stages; were estimated and assigned by comparison with the observed FTIR spectra.