Ideal gas thermodynamic properties, Delta H(f 298)degrees, S(f 298)degrees, and Cp(T) (300 less than or equal to T/K less than or equal to 1500), for all C-1 and C-2 and eight C-3-C-6 chlorinated hydrocarbons are calculated by AM1 and PM3 in MOPAC6 and then compared with the available literature data. The approximation method of Fitter and Gwinn is used to account for internal rotor contributions to S(298)degrees and Cp(T) (300 less than or equal to T/K less than or equal to 1500),in place of the torsion frequency estimation in MOPAC. AM1- and PM3-derived S(298)degrees and Cp(T) (300 less than or equal to T/K less than or equal to 1500) are found to be consistent with the literature data (relative deviation less than or equal to +/-5%) except S(298)degrees of C2HCl5 and Cp (300-500 K) of chloroacetylenes, Delta H(f 298)degrees of CCl4, C2HCl3, C2Cl4, and C2Cl2 obtained by AM1 and Delta H(f 298)degrees of CH2Cl2, CH2ClCH2Cl, CH2ClCHCl2, CHCl2CHCl2, C2Cl4, and C2Cl2 obtained by PM3 are not in good agreement with the literature values (deviation greater than or equal to 15 kcal/mol), Delta H(f 298)degrees (kcal/mol) calculated by PM3 are corrected as follows : Delta H(f 298,experiment)degrees = -2.35 + 1.086 Delta H(f 298,PM3)degrees for the partially chlorinated hydrocarbons; Delta H(f 298,experiment)degrees = 5.36 + 1066 Delta H(f 298,PM3)degrees for the fully chlorinated hydrocarbons. A modified group additivity method implemented in THERM is shown to be precise for predicting Delta H(f 298)degrees, S(298)degrees, and Cp(T) (300 less than or equal to T/K less than or equal to 1500) of C-1 and C-2 chlorocarbons. AM1- and PM3-calculated Delta H(f 298)degrees for chlorocarbons can be improved by linear correction equations. Comparison of estimations from group additivity, corrected MOPAC6/PM3, and literature data suggests that the modified group additivity is the better estimation method.