Hypochlorites are formed from reactions of chlorine monoxide (ClO) addition to unsaturates and association with radicals. ClO is a prevalent and an important species in the chemistry of the atmosphere and in combustion systems where chlorine is present. Thermochemical property data on these oxychlorocarbon species are important to understanding the reaction pathways and kinetics in these environments. Enthalpy, DeltaH(f298)degrees, entropy, S(298)degrees, and heat capacities, C-p(T) from 300 to 1500 K are determined for methyl hypochlorite and three chloromethyl hypochlorites 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/6-31G(d,p), B3LYP/6-311+G(3df,2p), QCISD(T)/6-31G(d,p), and CBS-Q levels of calculation. Enthalpies of formation are determined at each calculation level using the DeltaH(rxn)degrees and known enthalpies of other reactants in each of several working reactions (up to seven). Barriers for intramolecular rotation are calculated and contributions to entropy and heat capacity from internal rotation in the chloromethyl hypochlorite are determined. Evaluation of enthalpy data from reaction schemes and the statistical distribution of rotation conformers results in DeltaH(f298)degrees values for CH3OCl of -15.41 +/- 1.48 kcal/mol, CH2ClOCl of -22.05 +/- 2.47 kcal/mol, CHCl2OCl of -26.14 +/- 3.61 kcal/mol, and CCl3OCl of -26.72 +/- 4.68 kcal/mol in the CBS-Q//B3LYP/6-31G(d,p) method.