With the use of the method of crossing parabolas, the following reactions of hydrogen-atom abstraction by molecular oxygen were analyzed: (RCH2CHR1)-H-. + O-2 --> RCH = CHR1 + HO2., RCH2O. + O-2 --> RCH(O) + HO2., (RCHOH)-H-. + O-2 --> RCH(O) +HO2. The activation energies E-e0 for thermoneutral analogs of these reactions were calculated: E-e0 = 51.9 and 79.6 kJ mol(-1) for alkyl and alkoxyl radicals, respectively. An important role of polar interaction in the transition state is pointed out, which explains the difference between E-e0(RCH2O. + O-2) and E-e0((RCH2CHR1)-H-. + O-2). The contribution of polar interaction to the activation energy of the reaction RCH2O. + O-2 is 27.7 kJ mol(-1). The triplet repulsion also plays an important role, thus explaining the difference in E-e0 for reactions of alkyl and ketyl radicals with oxygen.