The temperature dependence of the standard molar heat capacity C-p,m(o) of samples of crystalline tetraphenylphosphonium perchlorate and tetraphenylarsonium perchlorate was measured in an adiabatic low-pressure calorimeter between T = 4.8 K and T = 340 K and from T = 5.8 K to T = 340 K, respectively, mostly to within a precision of 0.2 per cent. For tetraphenylphosphonium perchlorate, an anomalous change of the heat capacity in the range T = 125 K to T = 185 K, probably arising from the excitation of hindered rotations of atomic groups, was found and its thermodynamic characteristics were determined. No such anomaly was observed for tetraphenylarsonium perchlorate. The data obtained were used to calculate the thermodynamic functions C-p,m(o) (T)/R, Delta (T)(0) H-m(o)/R.K, Delta (T)(0) S-m(o)/R, and Phi (o)(m) = Delta S-T(0)m(o) - Delta H-T(0)m(o)/T(where R is the universal gas constant) of the compounds between T -->0 and T = 340 K.