The enthalpies of formation (Delta(f)H(298)) of biphenyl, 2,2-and 4.4'-dichlorobiphenyls were calculated at the B3LYP density functional theory (DFT) level using isodesmic reactions. The values for biphenyl and 4,4'-dichlorobiphenyl are in good agreement with calorimetric values, whereas, the values for 2.2'-dichlorobiphenyl are noticeably larger than the experimental one. The results of calculations allow to suggest that experimental value of Delta(f)H(298) for 2,2'-isomer is underestimated by 1-2 kcal/mol.