The electronic structure and spin density distribution of peroxyl radicals are investigated by density functional theory (DET) at the B3LYP level. Results found for superoxide anion and tert-butyl peroxyl radicals at a variety of basis sets suggest that 6-31G is the most appropriate basis set for calculation of hyperfine coupling constants (hfcc＇s) of carbon-based peroxyl radicals. Calculation of parallel O-17 hfcc＇s [A(parallel to)(O-17)] for a series of substituted methyl peroxyl radicals with the 6-31G basis set yielded calculated values with a maximum deviation of 2.2% from experiment. Spin density distributions estimated from experiment A(parallel to)(O-17) are compared to theoretical estimates from Mulliken orbital population analysis. Electronegative substitution at the carbon alpha to the peroxyl group results in an increase of terminal oxygen hyperfine coupling and spin density, shortening of C-O, and lengthening of O-O. In cases involving significant steric hindrance, however, C-O bond shortening was prevented. A(parallel to)(O-17) values for the terminal peroxyl oxygen atom correlate well with Taft sigma* substitutent parameters for the R group in the peroxyl radicals (ROO.). Thiyl peroxyl radicals are reinvestigated using B3LYP for comparison to previous theoretical work at UHF level. This resulted in confirmation that the effect of the addition of an electron pair donor (hydroxide ion) to CH3SOO. is to alter the spin density distribution in the peroxyl group. Structural models of lipid peroxyl radicals show that vinyl peroxyl radicals may be distinguished from saturated, allylic, and ester-based peroxyl radicals on the basis of hyperfine coupling constants.