The heat capacity C-p(T) of leucite with chemical composition (K0.91Na0.04Ca0.01)[Al0.97Si2.03O6].0.084H(2)O from Orvieto. Italy was measured in a vacuum adiabatic calorimeter in the temperature range 5.9-310 K. No anomalies were detected in the C-p(T) dependence. Calculated values of heat capacity and thermodynamic functions at standard conditions are as follows: C(p)degrees(298.15) = 166.4 +/- 0.2 J mol(-1) K-1; Sdegrees(298.15) = 182.4 +/- 0.3 J mol(-1) K-1; Hdegrees (298.15) - Hdegrees (0) = 28120 +/- 40 J mol(-1); Phidegrees(298.15) = 88.04 +/- 0.3 J mol(-1) K-1. These results are in good agreement with previous measurements above 51 K [US Bur. Mines Rep. Invest, 4955 (1992) 22]. Recommended values of heat capacity and absolute entropy at standard conditions for stoichiometric leucite KAISi(2)O(6) are: C(p)degrees (298.15) = 163.8 +/- 1.0 J mol(-1) K-1; Sdegrees (298.15) = 180.0 +/- 1.0 J mol(-1) K-1.