The thermal decomposition and isomerization processes of C3-C4 alkyl radicals, 1-C5H11, and 1-C6H13 have been investigated by using a shock-tube apparatus coupled with atomic resonance absorption spectrometry (ARAS). Isomeric alkyl radicals were generated by the thermal decomposition of respective alkyl iodides. Branching fractions for the competitive pathways (C-C bond cleavage, C-H bond cleavage, and isomerization) have been determined by following the hydrogen-atom concentration by ARAS. In the investigated temperature range (900-1400 K), for all alkyl radicals, the energetically favored C-C bond cleavage was found to dominate over the C-H bond cleavage. The 1,2 or 1,3 isomerization reaction was found to be minor in C3 and C4 alkyl radicals. On the other hand, the results for 1-C5H11 and 1-C6H13 radicals clearly show the occurrence of 1,4 and 1,5 isomerization reactions. From an RRKM analysis of the present result and the previous lower temperature data, with consideration of the tunneling effect, the threshold energies for 1,4 and 1,5 primary-to-secondary isomerization reactions were evaluated to be 21.5 ± 1.2 and 14.6 ± 1.2 kcal mol-1, respectively. The high-pressure limit rate constants for the isomerization processes were evaluated as k∞(1-C5H11 → 2-C5H11) = 4.88 × 108T0.846 exp(-19.53 [kcal mol-1]/RT) s-1 and k∞(1-C6H13 → 2-C6H13) = 6.65 × 107T0.823 exp(-12.45 [kcal mol-1]/RT) s-1 for the temperature range 350-1300 K. Even under relatively high-pressure conditions (∼1 atm), the falloff effect was shown to be important for multichannel dissociation systems. The nonequilibrium effect in the thermal decomposition of energized alkyl radicals formed in the high-temperature reaction system, which has been first suggested by Tsang et al. [J. Phys. Chem. 1996, 100, 4011] was discussed. The possible effect of the tunneling in the isomerization reactions was discussed in comparison with previous lower temperature data.