The heat capacity of a halogen-bridged quasi-one-dimensional (Q1D) mixed-valence binuclear metal complex (the so-called MMX chain), Pt-2(n-PrCS2)(4)I, was measured by adiabatic calorimetry. A higher order phase transition due to structural disorder was observed at 209 K. Another first-order phase transition having a large tail on the low-temperature side was detected at 358.8 K. Their enthalpy and entropy of transition were determined and analyzed. As evidenced by the magnitude of the entropy of transition, similar structural disorder occurs at the corresponding phase transition in Pt-2(n-PrCS2)(4)I and Pt-2(n-BuCS2)(4)I, but the shape of thermal anomaly is largely different between the two. The magnitude of the entropy of transition shows that the change of both structural disorder and electronic states occurs simultaneously in a gradual manner on the low-temperature tail of the thermal anomaly at 358.8 K. The change in entropy assignable to the Q1D electron system was estimated as about 2.3 J K-1 mol(-1).