The standard enthalpies of formation of Bi25FeO39 (s) and Bi2Fe4O9 (s), at 298.15 K were determined by acid solution calorimetry and are found to be (-7.55 +/- 0.32) x 10(3) kJ mol(-1) and (-2.17 +/- 0.07) x 10(3) kJ mol(-1), respectively. Heat capacities of Bi25FeO39 (s) and Bi2Fe4O9 (s) were measured using differential scanning calorimetry (DSC). The measured heat capacities as a function of temperature are expressed as: C-p < Bi25FeO39> (JK(-1) mol(-1)) +/- 49.9 = 1529.9 + 200.6 x 10(-3) T - 21.6 x 10(6) T-2 (T/K: 308-818), C-p (JK(-1) mol(-1)) +/- 11.0 = 395.5 x 23.0 x 10(-3) T - 7.6 x 106 T-2 (T/K: 313-773). Deviation of measured C-p values from the estimates by the Neumann-Kopp rule (NKR) has also been discussed. (C) 2017 Elsevier Ltd.