The standard (p degrees = 0.1 MPa) molar enthalpies of formation for 2-, 3-, and 4-phenylpyridine in the, gas phase were derived from the standard molar enthalpies of combustion, in oxygen, at 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of vaporization for 2-, 3-, and 4-phenylpyridine at T = 298.15 K were measured by correlation-gas chromatography. The enthalpy of sublimation of 4-phenylpyridine was obtained as a weighted mean of the value derived from the vaporization and fusion enthalpy values and the value measured directly by Calvet microcalorimetry. The following enthalpies of formation were then derived: 2-phenylpyridine, g = 228.3 +/- 5.8 kJ.mol(-1); 3-phenylpyridine, g = 240.9 +/- 5.5 kJ.mol(-1); 4-phenylpyridine, g = 240.0 +/-33 kJ.mol(-1). The most stable geometries of all phenylpyridine isomers were obtained using both restricted Hartree-Fock (RHF) and density functional theory (DFT/B3LYP) methods. The resulting geometries were then used to obtain estimates of enthalpies of formation of the three isomers of phenylpyridine, which are in good agreement with the experimental values. A theoretical interpretation of the effect of the phenyl ring has on the relative stabilities of the three molecules is presented.