Ab initio molecular energies at several levels of theory (MP4/6-311G**//MP2/6-31G*, MP4/6-311-G**//MP2/6-31G*,G1, and G2) are used to determine the heats of formation of several chlorinated hydrocarbons (allyl chloride, cis- and trans-1-chloropropene, and vinyl chloride) from atomization and isodesmic reactions. More than one isodesmic reaction was investigated for each molecule. Inconsistencies between the results from isodesmic reactions for a given molecule indicated possible errors in the experimental heats of formation for some of the chlorinated molecules used as references in the isodesmic reactions (in particular 1-chloropropane and 2-chloropropane). To further examine this possibility we did a multivariate regression for the G2 calculated reaction enthalpies for the 30 isodesmic reactions. In the regression, the heats of formation of the hydrocarbons and CH3Cl were fixed at the experimental values. The heats of formation of all the other chlorinated hydrocarbons were varied. The heats of formation determined using this method were : Delta H-f(298)(CH2Cl2)=-22.6 kcal/mole, Delta H-f(298)(CHCl=Cl-2)=5.0 kcal/mole, Delta H-f(298)(CCl2=CH2)=-0.2 kcal/mole, Delta H-f(298)(CH2Cl-CH3)=-27.0 kcal/mole, Delta H-f(298)(c-CHCl=CH-CH3)=-3.1 kcal/mole, Delta H-f(298)(t-CHCl=CH-CH3)=-2.8 kcal/mole, Delta H-f(298)(CH2=CClCH3)=-5.4 kcal/mole, Delta H-f(298)(CH2=CH-CH2Cl)=-0.8 kcal/mole, Delta H-f(298)(CH2Cl-CH2-CH3)=-32.2 kcal/mole, Delta H-f(298)(CH3-CHCl-CH3)=-35.9 kcal/mole. The calculated heats of formation were used to derive the following Benson group enthalpy values : C-(Cl)(H)2(C-d)=-15.6 kcal/mole, C-d-(Cl)(H)=-1.3 kcal/mole, and cis-halogen-alkyl=-0.3 kcal/mole.