The reactions of atomic O (P-3) with (CH3)(2)SiH2 and (CH3)(3)SiH have been studied theoretically using ab initio molecular orbital theory for the first time. Geometries have been optimized at the MP2 level with the 6-311G(d,p) and 6-311G(2d,2p) basis sets. The single-point energy calculations have been carried at the QCISD(T)/6-311+G(3df,2p) level. Theoretical analysis provides conclusive evidence that the main process occurring in each reaction is the hydrogen abstraction from the Si-H bonds leading to the formation of the H-2 and silyl radical; the hydrogen abstraction from the C-H bonds has higher barrier and is difficult to react. Two nearly degenerate transition states of (3)A' and (3)A' symmetries have been located for each hydrogen abstraction reaction from the Si-H bonds. Changes of geometries, generalized normal-mode vibrational frequencies, and potential energies along the reaction paths are discussed and compared. The rate constants have been deduced over a wide temperature range of 200-3000 K using canonical variational transition-state theory (CVT) with small curvature tunneling effect (SCT). The calculated CVT/SCT rate constants exhibit typical non-Arrhenius behavior, three-parameter rate-temperature formulas are fitted as follows (in units of cm(3) molecule(-1) s(-1)): k(1)(T)=(3.41x10(-16))T(1.65)exp(-411.72/T) and k(2)(T)=(1.85x10(-15))T-1.42 exp(-372.57/T) for the reactions of O (P-3) with (CH3)(2)SiH2 and (CH3)(3)SiH, respectively. The calculated rate constants are compared with the available experimental values. (C) 2003 American Institute of Physics.