The standard enthalpies of formation, Delta (f)H(298)degrees, were estimated for all 209 gaseous polychlorinated biphenyl (PCB) congeners combining the difference method - the procedure that is completely consistent with group additivity approach -and data on relative stability of PCB isomers modeled by the semiempirical method AM1 (Mulholland et al. J. Phys. Chem. 1993, 97, 6890). A set of 14 parameters was derived for estimating the enthalpy of formation values of PCBs by group additivity method. The results are compared with previous group additivity estimations. The proposed group additivity scheme is based on the experimental enthalpies of formation of biphenyl, 2, 2 ' -dichlorobiphenyl, 4,4 ' -dichlorobiphenyl, and multichlorinated benzenes. To check the doubt on the enthalpy of formation of 2,2 ' -dichlorobiphenyl, its value was determined by density functional calculations at the B3LYP level of theory using isodesmic reactions. The results of these calculations suggest that the experimental Delta (f)H(298)degrees value of 2,2 ' -dichlorobiphenyl is underestimated by at least 1-2 kcal/mel.