Thermochemical properties of oxygenated chlorocarbons are important in studies on oxidation and kinetics of chlorocarbons and in Cl initiated kinetic studies on oxidation of hydrocarbons. Enthalpy, DeltaH degrees (f98), entropy, S-298, and heat capacities, C-p(T), from 300 to 1500 K are determined for three chloro-dimethyl ethers by density functional and ab initio calculation methods. Molecular structures and vibration frequencies are determined at the B3LYP/6-31G(d,p) density functional calculation level, with single point calculations for energy at the B3LYP/6-311+G(3df,2p), QCISD(T)/6-31G(d,p), and CBS-Q//B3LYP/6-31G(d,p) levels of calculation (abbreviated as CBSQ//B3**). Enthalpies of formation are determined at each calculation level using the DeltaH(rxn(298)) and known enthalpies of other reactants in each of seven different working reactions. Contributions to entropy and heat capacity from internal rotation of the chloro-dimethyl ethers are also determined. Evaluation of data from three isodesmic schemes, and the energies of rotational conformers, result in DeltaH degrees (f298) values for CH2ClOCH3 of -55.42 +/- 0.91, for CHCl2OCH3 of -62.28 +/- 2.10, and for CCl3OCH3 of -63.11 +/-2.26 kcal/mol at the CBS-Q//B3** level. We determine enthalpy, entropy, and C-p(T) terms for the three mono-oxygenated carbon chlorine groups, where enthalpy values for use in group additivity are C/Cl/H2/O (-20.90), C/Cl2/H/O (-28.23), and C/Cl3/O (-28.69 kcal/mol), respectively.