Heat capacity of the halogen-bridged mixed-valence complex Pt-2(dta)(4)I (dta = CH3CS2-) has been measured in the temperature region between 6 and 386 K. The complex exhibited a phase transition of order-disorder type at T-trs = 373.4 K, from being arranged in an ordered helical form of four dta ligand planes around the central Pt-Pt axis to dynamically jumping between two orientations. Neither latent heat nor distinct thermal hysteresis was observed for the phase transition, suggesting that the phase transition is of higher-order rather than of first-order. The transition enthalpy and entropy were determined to be Delta(trs)H = 1.91 +/- 0.02 kJ mol(-1) and Delta(trs)S = 5.25 +/- 0.07 J K-1 mol(-1), respectively. The transition entropy close to R In 2 (= 5.76 J K-1 mol-1), where R is the gas constant, implies that the twisting motion of the four dta ligands takes place in a synchronized way. No thermal anomaly was detected around 300 and 90 K, where a Mott transition from the one-dimensional metallic phase to the semiconducting phase and a spin-Peierls-like transition have been expected. The reason for the absence of these transitions in calorimetry is discussed briefly.