Measurements leading to the calculation of standard entropies for 1,10-phenanthroline (Chemical Abstracts registry number [66-71-7]) in the crystal, liquid, and ideal-gas state are reported. Experimental methods were adiabatic heat-capacity calorimetry and comparative ebulliometry. Thermodynamic properties for phenanthridine [229-87-8] and 7,8-benzoquinoline [230-27-3] were reported previously and included those measured with adiabatic heat-capacity calorimetry, comparative ebulliometry (7,8-benzoquinoline only), inclined-piston manometry, and combustion calorimetry. New measurement results for phenanthridine and 7,8-benzoquinoline 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 (dsc), and vapor pressures by comparative ebulliometry (phenanthridine only). All critical properties were estimated. Molar entropies for the ideal-gas state were derived for all 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 the three compounds studied. These are shown to be in excellent accord with the calorimetric results for 1,10-phenanthroline and phenanthridine. Results for 7,8-benzoquinoline indicate that the crystal state is disordered. All new experimental results are compared with property values reported in the literature. Published by Elsevier Ltd.