Thermal decomposition of CH2I2 [sequential C-I bond fission processes, CH2I2 + Ar -> CH2I + I + Ar (1a) and CH2I + Ar -> (CH2)-C-3 + I + Ar (1b)], and the reactions of (CH2)-C-3 + H-2 -> CH3 H (2) and (CH2)-C-1 + H-2 -> CH3 + H (3) have been studied by using atomic resonance absorption spectrometry (ARAS) of I and H atoms behind reflected shock waves. Highly diluted CH2I2 (0.1-0.4 ppm) with/without excess H-2 (300 ppm) in Ar has been used so that the effect of the secondary reactions can be minimized. From the quantitative measurement of I atoms in the 0.1 ppm CH2I2 + Ar mixture over 1550-2010 K, it is confirmed that two-step sequential C-I bond fission processes of CH2I2, (la) and (1b), dominate over other product channels. The decomposition step (1b) is confirmed to be the rate determining process to produce 3CH2 and the least-squares analysis of the measured rate gives, In(k(1b)/cm(3) molecule(-1) s(-1)) = -(17.28 +/- 0.79) - (30.17 +/- 1.40) x 10(3)/T. By utilizing this result, we examine reactions 2 and 3 by monitoring evolution of H atoms in the 0.2-0.4 ppm CH2I2 + 300 ppm H2 mixtures over 18502040 K. By using a theoretical result on k(2) (Lu, K. W.; Matsui, H.; Huang, C.-L.; Raghunath, P.; Wang, N.-S.; Lin, M. C. J. Phys. Chem. A 2010, 114, 5493), we determine the rate for (3) as k(3)/cm(3) molecule(-1) s(-1) = (1.27 +/- 0.36) x 10(-10). The upper limit of k(3) (k(3max)) is also evaluated by assuming k(2) = 0, i.e., k(3max)/cm(3) molecule(-1) s(-1) = (2.26 +/- 0.59) x 10(-10). The present experimental results on k(3) and k(3max) is found to agree very well with the previous frequency modulation spectroscopy study (Friedrichs, G.; Wagner, H. G. Z. Phys. Chem. 2001, 215, 1601); i.e., the importance of the contribution of (CH2)-C-1 in the reaction of CH2 with H-2 at elevated temperature range is reconfirmed.