Thermodynamic properties of monoclinic Fe-2(SO4)(3) were measured by acid solution calorimetry, adiabatic calorimetry, and semi-adiabatic calorimetry. The standard molar enthalpy of formation Delta(f)H degrees(m) was determined as -(2585.2 +/- 4.9) kJ (.) mol(-1) by an appropriate thermochemical cycle with enthalpies of solution of monoclinic Fe-2(SO4)(3), alpha-MgSO4, gamma-FeOOH, H2O, and MgO in 5 N HCl at T = 298 K. Heat capacity data were collected from T = (0.5 to 400) K. The standard molar entropy S. at T = 298.15 K is (305.6 +/-0.6) J (.) K-1 (.) mol(-1). While the enthalpy of formation agrees well with a previous Delta(f)H degrees(m) determination, the entropy differs significantly because magnetic entropy was entirely overlooked in the previous work. The heat capacity shows a sharp magnetic transition with the peak located at 30.0 K. The C-p.m data in the temperature range (273 to 395) K are represented by a Maier-Kelley polynomial C-p,C-m/J. K-1 (.) mol(-1) = 213.0 + 0.3121(T/K) - 2.959 x 10(6()T/K)(-2). The calculated standard molar Gibbs free energy of formation from the elements Delta(f)G degrees(m) at T = 298.15 K is -(2264.5 +/- 4.9) kJ (.) mol(-1). (c) 2004 Elsevier Ltd. All rights reserved.