In a rubidium manganese hexacyanoferrate, RbMn[Fe(CN)(6)], the magnetic susceptibility (chi(M)) decreased at 225 K (= T1/2(down arrow)) and abruptly increased at 300 K (=T-1/2up arrow) in the cooling and warming processes, respectively. X-ray photoelectron spectroscopy and infrared spectroscopy indicated that the high-temperature (HT) and low-temperature (LT) phases were composed of Mn-II-NC-Fe-III and Mn-III-NC-Fe-II, respectively. A structural change from cubic (F43m, a = 10.533 Angstrom) to tetragonal (I4m2, a = b = 7.090 Angstrom, c = 10.520 Angstrom) accompanied the phase transition, and, on the basis of these results, the HT and LT phases were assigned to Mn-II(t(2g)(3)e(g)(2), (6)A(1g); S = 5/2)-NC-Fe-III (t(2g)(5), T-2(2g); S = 1/2) and Mn-III(e(g)(2)b(2g)(1)a1g(1), B-5(1g); S = 2)-NC-Fe-II (b(2g)(2)e(g)(4), (1)A(1g); S = 0), respectively. This phenomenon is caused by a metal-to-metal charge transfer from Mn-II to Fe-III and a Jahn-Teller distortion of the produced Mn-III ion. The reaction mechanism is discussed, considering the entropy difference between the HT and LT phases.