The mechanism of the reactions between the halogenated methylperoxy radicals, CHX2O2 (X = F, Cl), and NO is investigated by using ab initio and density functional quantum mechanical methods. Comparison is made with the mechanism of the CH3O2 + NO reaction. The most important energy minima in the potential energy surface are found to be the two conformers of the halogenated methyl peroxynitrite association adducts, CHX(2)OONOcp and CHX(2)OONOtp, and the halogenated methyl nitrates, CHX2ONO2. The latter are suggested to be formed through the one-step isomerization of the peroxynitrite adduct and may lead upon decomposition to carbonylated species, CX2O + HONO and CHXO + XNO2. The ambiguous issue of the unimolecular peroxynitrite to nitrate isomerization is reconsidered, and the possibility of a triplet transition state involvement in the ROONOtp <-> RONO2 rearrangement is examined. The overall calculations and the detailed correlation with the methyl system show the significant effect of the halogenation on the lowering of the entrance potential energy well which corresponds to the formation of the peroxynitrites. The increased attractive character of the potential energy surface found upon halogenation combined with the increased exothermicity of the CHX2O2 + NO -> CHX2O + NO2 reaction are suggested to be the important factors contributing to the enhanced reactivity of the halogenated reactions relative to CH3O2 + NO. The calculated heat of formation values indicate the large stabilization of,the fluorinated derivatives.