Chlorinated hydrocarbon groups plus non-next-nearest-neighbor interaction terms, which combine with the Benson group additivity method for estimation of thermodynamic properties (Delta H-f(o), S-o and C-p(T)(300-1500 K) on multichloro alkanes and alkenes, are developed. New chlorocarbon alkane and alkene Benson-type group values are derived from molecule systems where no chlorines are on the carbon adjacent to the carbon atom bonded to chlorine(s). A set of interaction terms for Delta H-f(o), S-o, and C-p(T)(300-1500 K) is derived to account for non-next-nearest-neighbor chlorine-chlorine interactions. These are derived from the experimental thermodynamic property data on 28 chlorinated hydrocarbons species and current hydrocarbon groups. Thermodynamic properties for representative multichloro alkanes and alkenes determined using this modified group additivity scheme are compared with literature data and show good agreement (Delta H-o,298(f) = +/-0.29 kcal/ mol, S-298(o) = +/-0.68 cal/mol.K, and C-p(T) = +/-0.23 cal/mol.K). The use of a limited number of interaction groups provides improved accuracy in calculation of thermodynamic properties for multichloro alkanes and alkenes when chlorines are on adjacent carbon atoms. We develop three multichloro Benson groups plus five interaction groups for chloroalkanes, and two groups plus five interaction groups for chloroalkenes. This non-next-nearest-neighbor interaction group approach accounts for 13.6 kcal/mol in standard enthalpy for hexachloroethane and 2.7 cal/mol.K in standard entropy for tetrachloroethylene. The multichloro groups combined with the interaction groups allow estimation of thermodynamic properties (Delta H-f(o), S-o, and C-p(T)-(300-1500 K)) on larger multichlorocarbon species, where no thermodynamic property data or accurate estimation techniques are available.