Chlorinated methyl hydroperoxides are important intermediates in the oxidation, combustion, and atmospheric photochemistry of chlorocarbons. The thermochemical property data on these oxy-chlorocarbon species are important for understanding their stability, reaction paths, and kinetics. Enthalpy, Delta H-f(298)o, entropy, S-298(o), and heat capacities, C-p(T) (300 less than or equal to T/K less than or equal to 1500), are determined for monochloromethyl hydroxyperoxide, dichloromethyl hydroperoxide, and trichloromethyl hydroperoxide using density functional B3LYP/6-31 G(d,p), and 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 (abbreviated as CBSQ//B3**). The 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/G-311+G(3df,2p), QCISD(T)/6-31G(d,p) and CBSQ//B3LYP/6-31G(d,p) levels. The vibration frequencies are scaled for zero point energies and for thermal corrections. The enthalpies of formation (Delta H-f(298)o) are determined at each calculation level using the Delta H-rxn,298(o) with known enthalpies of other reactants and products in each of five different reactions. Standard entropy (S-298(o)) and heat capacity (C-p(T)'s, 300 less than or equal to T/K less than or equal to 1500) from vibrational, translational, and external rotational contributions 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 rotation are calculated at the B3LYP/6-31G(d,p) level, and hindered internal rotational contributions to entropy and heat capacity are calculated by summation over the energy levels obtained by direct diagonalizations of the Hamiltonian matrix of hindered internal rotations. An evaluation of data from the reactions, several of which are isodesmic, results in Delta H-f(298)o values for CH2ClOOH of -41.41 +/- 1.45kcal/mol, CHCl2OOH of -44.74 +/-3.25 kcal/mol, and CCl3OOH of -45.63 +/- 3.14 kcal/mol. The Delta H-f(298)o values suggest that the electronegatlve Cl(s) on the methyl increase stability and ROO-H bond energy by several kcal/mol relative to CH3OOH. Groups for use in Benson type additivity estimations are determined for the carbon with oxygen and chlorine(s). The enthalpy values for the C/Cl/H-2/OO, C/Cl-2/H/OO and C/Cl-3/OO groups are -17.91, -21.24, and -22.13 kcal/mol respectively with error limits as above. The enthalpy values from reactions that are isodesmic show good agreement at all of the theory levels, suggesting effective cancellation of errors in the reaction sets. CBSQ//B3LYP/6-31G(d,p) calculations are judged to be the most accurate for enthalpies from nonisodesmic reactions, in this study.