Standard enthalpies of formation for the free radicals CH2OCl and CH2OBr were theoretically estimated using two hydrogenation and two isodesmic reactions as working chemical reactions. Energy differences were computed at two high ab initio levels of calculation, using GAUSSIAN 2 (G2) theory (Level I) and Truhlar's basis set limit method (Level II). The recommended standard enthalpies of formation (at 298 K and 1 arm) are the unweighted averages of the results obtained at these two levels with the four working chemical reactions, namely: CH2OCl, 32.0 +/- 3.5; and CH2OBr, 36.1 +/- 3.9 kcal mol(-1). These enthalpies of formation, correspond to bond dissociation enthalpies of DH298o (H-CH2OCl) = 97.3 +/- 5.3 and DH298o (H-CH2OBr) = 96.9 +/- 6.3 kcal mol(-1).