The competition between the C-I bond fission and the four-center HI elimination in the thermal unimolecular decomposition of C3-C4 alkyl iodides has been investigated at temperatures of 950-1400 K and pressures around 1 atm by a shock tube technique. The concentration of iodine atoms was followed by atomic resonance absorption spectrometry. For primary iodides, the absolute rate constants were measured at temperatures of 950-1100 K. The branching fractions for C-I bond fission channels were determined for all isomers of C3 and C4 alkyl iodides at temperatures of 950-1400 K. A drastic change in the branching fraction for the C-I bond fission channel was observed from primary iodides (0.6-0.9) to secondary iodides (0.2-0.4), and further to tertiary iodide (<0.05), which was mainly ascribed to the lowering of the threshold energy for the HI elimination channel from primary to secondary (by ∼14 kJ mol-1) and from secondary to tertiary (by ∼20 kJ mol-1) iodides. The α-CH3 substituent effect to the activation energy was in good accordance with previous investigations. The observed temperature dependence of the branching fraction could not be explained by the simple high-pressure limit treatment, and an RRKM analysis showed that the proper treatment of the mutual effect of two dissociation channels is essentially important to reproduce the observed branching fractions and their temperature dependence. A simple interpretation for the α-CH3 substituent effect is presented in terms of the avoided intersection between ionic dissociation (RI → R+ + I-) surface and the repulsive surface of HI approach to the double bond.