Heat capacities of PbBi12O19(s) and phi-Pb5Bi8O17(s) were measured by differential scanning calorimetry in the temperature range 280-820 K and 280-660 K, respectively. PbBi12O19(s) and phi-Pb5Bi8O17(s) showed order-disorder transitions whose onset temperatures are 520 K and 700 K, respectively. The measured heat capacities of PbBi12O19(s) and phi-Pb5Bi8O17(s) as a function of temperature are expressed as: C-p < PbBi12O19 > (J K-1 mol(-1)) = 791.68 + 112.18 x 10(-3) T - 13.58 x 10(6) T-2 (280 - 820 K) C-p < phi-Pb5Bi8O17 > (J K-1 mol(-1)) = 700.02 + 161.58 x 10(-3) T -12.85 x 10(6) T-2 (280 - 660 K) Enthalpy increments, entropies and Gibbs energy functions were derived from the measured heat capacity data. (C) 2005 Elsevier B.V. All rights reserved.