In the specific heat measurement of (RbMn[1])-Mn-1[Fe-11(CN)(6)] (M-[1] : S = 2, Fe-11; S = 0), an anomalous peak due to a long-range magnetic ordering was observed at 11.0 K (T-p). The magnetic transition entropy (DeltaS(mag)) and enthalpy (DeltaH(mag)) were evaluated from the magnetic specific heat (C-mag) to be 11.8 +/- 0.9 J K-1 mol(-1) and 125 +/- 9 J mol(-1), respectively. A detailed analysis of C-mag, DeltaS(mag), DeltaH(mag), and T-p has shown that the present magnetic phase is a three-dimensional Heisenberg-type ferromagnetic lattice of Mn-[1] sites with an exchange coupling constant of approximate to +0.5 cm(-1). The ferromagnetic ordering is ascribed to the valence delocalization mechanism. (C) 2004 Elsevier B.V. All rights reserved.