The thermal decompositions of isobutane and neopentane have been studied using both shock tube experiments and ab initio transition state theory based master equation calculations. Dissociation rate constants for these molecules have been measured at high temperatures (1260-1566 K) behind reflected shock waves using high-sensitivity H-ARAS detection. The two major dissociation channels at high temperature are iso-C4H10 -> CH 3 + i-C3H7 (la) and neo-C5H2 -> CH3 + t-C4H9 (2a). Ultrahigh-sensitivity ARAS detection of H-atoms produced from the rapid decomposition of the product radicals, i-C3H7 in (1a) and t-C4H9 in (2a), through i-C3H7 + M -> H + C3H6 + M (3a) and t-C4H9 + M -> H + i-C4H8 + M (4a) allowed measurements of both the total decomposition rate constants, k(total), and the branching to radical products, which were observed to be equivalent in both systems, k(1a)/k(total)., and k(2a)/k(total) = 0.79 +/- 0.05. Theoretical analyses indicate that in isobutane, the non-H-atom fraction has two contributions, the dominant fraction being due to the roaming radical mechanism leading to molecular products through iso-C4H10 -> CH4 + C3H6 (1b) with k(1b)/k(total) = 0.16, and a minor fraction that involves the isomerization of i-C3H7 to n-C3H7 that then subsequently forms methyl radicals, i-C3H7 + M -> n-C3H7 + M -> CH3 + C2H4 + M (3b). In contrast to isobutane, in neopentane, the contribution to the non-H-atom fraction is exclusively through the roaming radical mechanism that leads to neo-C5H2 --> CH4 + i-C4H8 (2b) with k(2b)/k(total) = 0.21. These quantitative measurements of larger contributions from the roaming mechanism for larger molecules are in agreement with the qualitative theoretical arguments that suggest long-range dispersion interactions (which become increasingly important for larger molecules) may enhance roaming.