The direct hydrogen abstraction reactions of H atom with GeH4, CH3GeH3, (CH3)(2)GeH2, and (CH3)(3)GeH have been studied systematically using ab initio molecular orbital theory. Geometries have been optimized at the UMP2 level with 6-31G(d) and 6-311G(2df,p) basis sets. G2MP2 theory has been used in the final single-point energy calculation. Theoretical analysis provided conclusive evidence that the main process occurring in each case is the hydrogen abstraction from the Ge-H bond leading to the formation of the H-2 and germyl radicals; the hydrogen abstraction from the C-H bond has higher barriers and is difficult to react. The kinetic calculations of the title reactions have been deduced using the canonical variational transition-state theory (CVT) with the small-curvature tunneling correction method (SCT) over the temperature range of 200-3000 . The CVT/SCT rate constants exhibit typical non-Arrhenius behavior. Three-parameter rate-temperature formulas have been fitted as follows: k(1) = (2.17 x 10(-17))T-2.16 exp(-294.2/T), k(2) = (2.21 X 10(-17))T-2.22 exp(-161.6/7), k(3) = (1.96 x 10(-17))T-2.18 exp(- 108.0/T), and k(4) = (6.66 x 10(-18))T-2.33 exp(-60.3/T) for the reactions of H with GeH4, CH3GeH3, (CH3)(2)GeH2, and (CH3)(3)GeH, respectively (in units of cm(3) molecule(-1) s(-1)). Studies show that the methyl substitution has an effect on the strength and reactivity of the Ge-H bond in (CH3)((4-n))GeHn (n = 1-3). The calculated CVT/SCT rate constants are in excellent agreement with the available experimental values.