Thermochemical property data on chlorinated methanols and methyl hydroperoxides are important in oxidation, combustion and atmospheric photochemistry of chlorocarbons, Enthalpy, entropy, and heat capacities are determined for three chlorinated methanols and three chlorinated methyl hydroperoxides using density frnctional, B3LYP/6-31G(d,p), B3LYP/6-311+G(3df,2p), ab initio QCISD(T)/6-31G(d,p) and the composite CBSQ/B3LYP/6-31G(d,p) calculation methods. Molecular structures and vibration frequencies are determined at the B3LYP/6-31G(d,p) density functional calculation level. Vibration frequencies are scaled for zero point energies and for thermal corrections. ΔH°_{f 298} are determined at each calculation level using the ΔH_rxn,298 with known enthalpies of other reactants and products in each of five different reactions. S°₂₉₈ and C_p(T) contributions from vibrations, translations, and external rotations are calculated using the rigid-rotor-harmonic-oscillator approximation based on the vibration frequencies and structures obtained from the density functional studies. Potential barriers for internal rotations are calculated at B3LYP/6-3 1G(d,p) level. Groups for use in Benson type additivity estimations are determined for the carbon with oxygen and chlorine(s). Enthalpy values from the isodesmic reaction show good agreement at all theory levels, suggesting effective cancellation of errors for these two series of molecules.