Heat capacities of the disordered and ordered crystalline phases of p-chloronitorobenzene were measured by adiabatic calorimetry between 6 and 303 K. The disordered phase stable at room temperature shows a glass transition around 245 K, which arises from the freezing of molecular reorientation as evidenced from the magnitude of the residual entropy. The ordered phase that is stable below 279 K was obtained through annealing the disordered phase around 260 K for a long time. The thermodynamic quantities concerning the first-order phase transition between the ordered and disordered phases were determined as T-trs=279.0 K, Delta(trs)H=(4468 +/- 5) J mol(-1) and Delta(trs)S=(16.01 +/- 0.05) J K-1 mol(-1). Standard thermodynamic functions are tabulated.