The heat capacity of ammonium hexabromoplatinate (NH4)(2)PtBr6 was measured from T = 6 K to T = 320 K by using adiabatic calorimetry. One lambda-shaped anomaly was found in the curve for heat capacity against temperature. This lambda-shaped transition reaches its maximum C-p,C- (m) = 369.3.R at T = (57.4 +/- 0.1) K, with Delta(trs)S(m)degrees = (2.491 +/-0.028).R characteristic of an order-disorder transition. From our results, the standard molar entropy Delta(T)(0)S(m)degrees{(NH4)(2) PtBr6, cr}/R = (57.36 +/-0.10), where T = 298.15 K and R = 8.314515 J.K.mol(-1). On the basis of this result, the free energy function Phi(m)degrees{(NH4)(2)PtBr6, cr}/R = (33.01 +/- 0.06). Smoothed values of the standard thermodynamic quantities for pure (NH4)(2)PtBr6 are tabulated at temperatures up to T = 320 K. 6 1998 Academic Press.