Chlorinated formyl methyl radicals ((CC)-C-.=O) are the stable keto forms of chlorovinoxy radicals formed by cleavage or astraction of the weak O-H bonds of chlorovinylalcohols. Thermochemical properties, DeltaH(f)degrees(298), Sdegrees(298), and C(p)degrees(T) (5 K less than or equal to T less than or equal to 6000 K), are computed by density functional B3LYP/6-31G(d,p) and B3LYP/ 6-311+G(3df,2p), ab initio QCISD(T)/6-31G(d,p), and composite CBS-Q calculation methods for chlorinated aldehydes and the corresponding chlorinated acetyl and formyl methyl radicals: CH3CHO, (1), CH3CClO (2), CH2ClCHO (3), CH2ClCClO (4), CHCl2CHO (5), CHCL2CClO (6), CCl3CHO (7), CCl3CClO (8), CH3C.=O (9), CH2ClC.=O (10), CHCl2C.=O (11), CCl3C.=O (12), (CH2CH)-H-.=O (13), (CH2CCl)-H-.=O (14), E-(CHClCH)-H-.= O (15), Z-(CHClCH)-H-.=O (16), (CCl2CH)-Cl-.=O (17), E-(CHClCCl)-H-.=O (18), Z-(CHClCCl)-H-.=O (19), (CCl2CCl)-Cl-.=O (20). Molecular structures and vibration frequencies are determined at the B3LYP/6-31 G(d,p) level of theory. Vibration frequencies are scaled for zero-point energies and thermal corrections. Two to four isodesn& reactions are utilized at each calculation level to determine DeltaH(f)degrees(298) of each species. Contributions to Sdegrees(298) and C(p)degrees(T) from translation, vibration, and external rotations are calculated using the rigid-rotor-hannonic-oscillator approximation based on the 133LYP/6-3 IG(d,p) structures. Hindered internal rotational contributions to entropies and heat capacities are calculated by summation over the energy levels obtained from direct diagonalizations of the Hamiltonian matrix of the internal rotation. The bond energies of C-H and C-Cl in chloroaldehydes are also calculated.