In this work, the aromaticity of pyracylene (2) was investigated from an energetic point of view. The standard enthalpy of hydrogenation of acenaphthylene (1) to acenaphthene (3) at 298.15 K was determined to be -(114.5+/-4.2) kJ mol(-1) in toluene solution and -(107.9+/-4.2) kJ mol-l in the gas phase, by combining results of combustion and reaction-solution calorimetry. A direct calorimetric measurement of the standard enthalpy of hydrogenation of pyracylene (2) to pyracene (4) in toluene at 298.15 K gave -(249.9+/-4.6) kJ mol-1. The corresponding enthalpy of hydrogenation in the gas phase, computed from the Delta(f)H(m)(o)(cr) and Delta(sub)H(m)(o) values obtained in this work for 2 and 4, was -(236.0+/-7.0) kJ mol-1. Molecular mechanics calculations (MM3) led to Delta(hyd)H(m)(o)(1,g) = -110.9 kJ mol(-1) and Delta(hyd)H(m)(o)(2,g) = -249.3 kJ mol(-1) at 298.15 K. Density functional theory calculations [B3LYP/6-311+G(3d,2p)//B3LYP/6-31G(d)] provided Delta(hyd)H(m)(o)(2,g) = -(244.6+/-8.9) kJ mol(-1) at 298.15 K. The results are put in perspective with discussions concerning the "aromaticity" of pyracylene. It is concluded that, on energetic grounds, pyracylene is a borderline case in terms of aromaticity/antiaromaticity character.