Measurements leading to the calculation of thermodynamic properties for phenazine (Chemical Abstracts registry number [92-82-0]) in the ideal-gas state are reported. Experimental methods included adiabatic heat-capacity calorimetry, inclined-piston manometry, and combustion calorimetry. Thermodynamic properties for acridine (Chemical Abstracts registry number [260-94-6]) were reported previously and included those measured with adiabatic heat-capacity calorimetry, comparative ebulliometry, inclined-piston manometry, and combustion calorimetry. New measurement results for acridine reported here are densities determined with a vibrating-tube densimeter and heat capacities for the liquid phase at saturation pressure determined with a differential-scanning calorimeter (d.s.c.). All critical properties were estimated. Molar entropies for the ideal-gas state were derived for both compounds at selected temperatures. Independent calculations of entropies for the ideal-gas state were performed at the B3LYP/6-31+G(d, p) model chemistry for phenazine and acridine. These are shown to be in excellent accord with the calorimetric results. All results are compared with experimental property values reported in the literature. Published by Elsevier Ltd.