Earlier experimental results on the kinetics of relaxation to equilibrium in R + O-2 reversible arrow RO2 reactions (R = CH3, C2H5, i-C3H7, t-C4H9, CH2Cl, CHCl2, and CCl3) are reanalyzed using an improved kinetic mechanism which accounts for further reactions of the RO2 adduct. Reaction enthalpy Delta H-298(o)(R + O-2 reversible arrow RO2) values are obtained from the third-law treatment of the temperature dependencies of the equilibrium constants K-p(T) resulting from the reinterpretation of the original kinetic data. The R-O-2 bond strengths for alkyl and chloroalkyl radicals (R = . C(CH3)(i)CljHk, i + j + k = 3) can be represented by a linear function of the numbers of C-C (NC-Cl) and C-Cl (NC-Cl) bonds at the C atom forming the C-O bond : -Delta H-298(o)(R + O-2 reversible arrow RO2) = H-CH3 + h(C-C)N(C-C) + h(C-Cl)N(C-Cl) (H-CH3 = 139.9, h(C-Cl)= 5.7, h(C-Cl) = -16.1 kJ mol(-1)). The values of Delta H-298(o)(R + O-2 reversible arrow RO2) are used to calculate the enthalpies of formation for the RO2 peroxy radicals and ROOH hydroperoxides.