Standard enthalpies of combustion of phenanthrene and naphthacene were determined to be - (7048.7 +/- 0.9) kJ (.) mol(-1) and - (9005.1 +/- 1.8) kJ (.) mol(-1), respectively, by means of high-precision micro-combustion calorimetry. The former result agrees very well with that reported by Steele et al. (AIChE Symposium Series 279, 1990, 86, 138-154). The latter confirms theoretical predictions. By using standard enthalpies of sublimation from the literature, the standard molar enthalpies of formation of phenanthrene and naphthacene in the gas phase were derived to be (201.7 +/- 2.9) kJ (.) mol(-1) and (331.6 +/- 4.4) kJ (.) mol(-1), respectively. The enthalpy of the rearrangement reaction from anthracene to phenanthrene was evaluated to be -(28.4 +/- 4.1) kJ (.) mol(-1).