Structures, internal rotational barriers, and thermochemical properties of 2-chloroethyl hydroperoxide, 2,2-dichloroethyl hydroperoxide, and 2,2,2-trichloroethyl hydroperoxide are computed by ab initio and density functional calculations. Molecular structures and vibrational frequencies are determined at the B3LYP/6-31G(d,p) density functional level, with single-point calculations for the energy at the B3LYP/6-311+G(3df,-2p), QCISD(T)/6-31G(d,p), and CBSQ//B3LYP/6-31G(d,p) levels. The Sdegrees(298) and C-p(T) values (0 less than or equal to T/K less than or equal to 5000) from vibrational, translational, and external rotational contributions are calculated using statistical mechanics based on the vibrational frequencies and structures obtained from the density functional study. Potential barriers for the internal rotations are calculated at the B3LYP/6-31G(d,p) level, and all minima and maxima on the torsional potentials are fully optimized. The hindered rotational contributions to Sdegrees(298) and C-p(T) are calculated by using direct integration over energy levels of the internal rotational potentials. The enthalpies of formation are calculated using isodesmic reactions, and the recommended DeltaH(f)degrees(298) values for CH2ClCH2OOH, CHCl2CH2OOH, and CCl3CH2OOH are -45.47 +/- 1.20, -48.92 +/-1.50, and -50.21 +/- 1.36 kcal/mol, respectively. Interaction terms for a peroxy group with chlorine(s) on a beta carbon are developed for the group additivity approach. Bond energies calculated from the enthalpies of beta-chlorinated ethyl hydroperoxides and their corresponding radicals show good agreement with those from our previous studies.