Standard reduction potentials, SRPs, of the halogen atoms have been calculated in water on the basis of an appropriate thermochemical cycle. Using the best up-to-date thermodynamic data available in the literature, we have calculated E degrees(X center dot/X-) values of 3.66, 2.59, 2.04, and 1.37 V vs SHE for F-center dot, Cl-center dot, Br-center dot, and I-center dot, respectively. Additionally, we have computed the SRPs of Cl-center dot, Br-center dot, and I-center dot in acetonitrile (CH3CN) and dimethylformamide (DMF) by correcting the values obtained in water for the free energies of transfer of X-center dot and X- from water to the nonaqueous solvent S and the intersolvent potential between water and S. From the values of E degrees(X center dot/X-) in CH3CN and DMF, the SRPs of a series of alkyl halides of relevance to atom transfer radical polymerization and other important processes such as pollution abatement have been calculated in these two solvents. This has been done with the aid of a thermochemical cycle involving the gas-phase homolytic dissociation of the C-X bond, solvation of RX, R-center dot, and X-center dot, and reduction of X-center dot to X- in solution.