Thermochemical kinetic analysis for the reactions of HO2 radical addition to the primary, secondary, and tertiary carbon-carbon double bonds of ethylene, propene, and isobutene are studied using canonical transition state theory (TST). Thermochemical properties of reactants, alkyl hydroperoxides (R . OOH), hydroperoxy alkyl radicals (R . OOH), and transition states (TSs) are determined by ab initio and density functional calculations. Enthalpies of formation (Delta H(f)degrees(298)) of product radicals (R . OOH) are determined using isodesmic reactions with group balance at MP4(full)/6-31G(d,p)/MP2(full)/6-31G(d), MP2(full)/6-31G(d), complete basis set model chemistry (CBS-q with MP2(full)/6-31g(d) and B3LYP/6-31g(d) optimized geometries), and density functional (B3LYP/6-31g(d) and B3LYP/6-311 + g(3df,2p)//B3LYP/6-31g(d)) calculations. Delta H(f)degrees(298) Of TSs are obtained from the Delta H(f)degrees(298) of reactants plus energy differences between reactants and TSs. Entropies (S degrees(298)) and heat capacities (Cp(T) 300 less than or equal to T/K less than or equal to 1500) contributions from vibrational, translational, and external rotational are calculated using the rigid-rotor-harmonic-oscillator approximation based on geometric parameters and vibrational frequencies obtained at MP2(full)/6-31G(d) and B3LYP/6-31G(d) levels of theory. Selected potential barriers of internal rotations for hydroperoxy alkyl radicals and TSs are calculated at MP2(full)/6-31G(d) and CBS-Q//MP2(full)/6-31G(d) levels. Contributions from hindered rotors of S degrees(298) and Cp(T) are calculated by the method of Pitzer and Gwinn and by summation over the energy levels obtained by direct diagonalization of the Hamiltonian matrix of hindered internal rotations when the potential barriers of internal rotations are available. Calculated rate constants obtained at CBS-q//MP2(full)/G-31G(d)and CBS-q//B3LYP/G-31G(d) levels of theory show similar trends with experimental data: HO2 radical addition to the tertiary carbon-carbon double bond (HO2 addition at CD/C2 carbon atom of isobutene) has a lower activation energy than addition to the secondary carbon-carbon double bond CD/C/H, which is lower than addition to the primary carbon-carbon bond CD/H2; the values are 12.11(11.56), 11.08(10.34), and 7.63(7.03) kcal/mol, respectively, at CBS-q/MP2 (full)/6-31G(d) level. Data in parentheses are calculations at the CBS-q//B3LYP/6-31G(d) level. The E-a for addition to primary carbon-carbon double bonds of ethylene, propene, and isobutene also show a decreasing trend 13.49(12.89), 12.16(11.20), and 10.70(10.59) kcal/mol, respectively. The high-pressure limit rate constants are (based on CBS-q//MP2(full)/6-31G(d) calculations) (Q = OOH): k(1,infinity)(HO2 + C=C double right arrow C . CQ) = 4.13 x 10(4) T-2.332 exp(-13.49 kcal mol(-1)/RT cm(3)/mol-s; k(2,infinity)(HO2 + C=CC double right arrow CC . CQ) = 2.47 x 10(4) T-2.132 exp(-12.16 kcal mol(-1)/RT) cm(3)/mol-s; k(3,infinity)(HO2 + C=CC double right arrow C . CQC) = 7.74 x 10(3) T(2.285)exp- (-11.08 kcal mol(-1)/RT) cm(3)/mol-s; k(4,infinity)(HO2 + C2C=C double right arrow C2C . CQ) = 3.45 x 10(5) T-1.765 exp(-10.70 kcal mol(-1)/RT) cm(3)/mol-s; k(5,infinity)(HO2 + C2C=C double right arrow C-3. CQ) - 2.78 x 10(4) T-2.106 exp(-7. 63 kcal mol(-1/)RT) cm(3)/mol-s, Enthalpies of formation for alkyl hydroperoxides are determined to be Delta H(f)degrees(298) (CCQ)= -39.70 +/- 0.3 kcal/mol, Delta H(f)degrees(298) (CCCQ = -44.77 +/- 0.41 kcal/mol, Delta H(f)degrees(298) (CCQC) = -48.99 +/- 0.32 kcal/mol, Delta H(f)degrees(298) (C(2)CCQ) = -51.32 +/- 0.38 kcal/mol, and Delta H(f)degrees(298) (C(3)CQ) = -57.91 +/- 0.47 kcal/mol. Delta H(f)degrees(298) of hydroperoxy alkyl radicals are Delta H(f)degrees(298) (C . CQ) = 10.96 +/- 1.06 kcal/mol, Delta H(f)degrees(298) (CC . CQ) = 2.62 +/- 1.29 kcal/mol, Delta H(f)degrees(298) (C . CQC) = 0.68 +/- 1.54 kcal/mol, Delta H(f)degrees(298) (C2C . CQ) = -7.24 +/- 1.25 kcal/mol, and Delta H(f)degrees(298) (C-3. CQ)= -6.42 +/- 1.28 kcal/mol.