Structures and thermochemical properties, DeltaH(f298)degrees, S(298)degrees, and C-p(T) of three (mono-, di-, and tri-) chloromethanols; three chloromethoxy and the two hydroxychloromethyl radicals are determined by ab initio and density functional calculations. The molecular structures and vibration 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 vibration frequencies are scaled for zero-point energies and for thermal corrections. The enthalpies of formation (DeltaH(f298)degrees) are determined at each calculation level using several isodesmic reactions. Standard entropy (S(298)degrees) 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 study. Potential barriers for internal rotation of the hydroxyl group 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 diagonalization on the Hamiltonian matrix of hindered internal rotations. An evaluation of data from the isodesmic reactions at each calculation level results in the enthalpy of formation values: -58.07 +/- 0.69, -65.88 +/- 0.76, and -65.96 +/- 0.76 kcal/mol for mono-, di-, and trichloromethanol, respectively. The standard enthalpies for the mono-, di-, and trichloromethoxy radicals are -5.13 +/- 2.18, -7.65 +/- 2.25, and -9.05 +/- 2.24 kcal/mol, respectively. The standard enthalpies for the hydroxymono- and hydroxydichloromethyl radicals are -14.46 +/- 1.75 and -22.54 +/- 1.83 kcal/mol. Bond energies for the RO-H bond are 105.04, 110.33, and 109.01 kcal/mol, respectively. Bond energies for the R-OH bonds are 95.20, 98.81, and 94.39 kcal/mol, respectively. Groups for use in Benson type additivity estimations are determined for the carbon bonded to oxygen and chlorine(s). The enthalpy values for the C/Cl/H-2/O, C/Cl-2/H/O, and C/Cl-3/O groups are -20.17, -27.98, and -28.06 kcal/mol, respectively. Hydrogen bond increment groups for the chloromethoxy and hydroxychloromethyl radicals are also developed. The bond energies and DeltaH(f298)degrees, values suggest that the electronegative Cl(s) on the methyl serve to increase and RO-H bond energy. DeltaH(f298)degrees, for (CH2OH)-H-., (CH3CHOH)-H-., and C2H5O. are also determined and compared with literature data, and recommended values are -3.97 +/- 0.22, -13.34 +/-: 0.84, and -3.90 +/- 1.27 kcal/mol, respectively.