The low-temperature heat capacities from 5 K to 330 or 350 K of acenaphthene (C12H10) and acenaphthytene (C12H8) have been determined by adiabatic calorimetry. Although the heat capacity of acenaphthene is typically sigmoidal, that of acenaphthylene has two transitions, one designated crystal phase III/crystal phase II (a rounded bump with a maximum at 116.6 K), and the other crystal phase II/crystal phase I (a very sharp spike located at 127.1 K). Severe hysteresis was noted on cooling through the regions of the two transitions, but optimized cooling permitted reproducible results for the thermodynamics of both transitions. The total Delta(trs)S degrees is 12.13 J.K-1.mol(-1) and approximates R ln 4. A comparison between the calorimetric and statistically estimated standard entropies in the ideal gas state for both compounds is made. Although a good agreement for acenaphthene is found at 300 K, evidence of residual entropy is found for acenaphthylene, estimated as about 10.33 J.K-1.mol(-1) (R ln 4). A correlation of these data based on neutron diffraction, NMR, and other studies will be shown. The thermodynamic functions C-p(J.K-1.mol(-1)), S degrees(J.K-1.mol(-1)), and phi degrees(J.K-1.mol(-1)) at 298.15 K are for C(12)H(8)183.59, 206.23, and 105.86 and for C12H10 191.38, 188.82, and 93.97.