Ab initio quantum mechanical methods were employed to study the H2SiO and HSiOH (both cis and trans) isomers, resulting in high-level theoretical predictions of the equilibrium geometries, rotational constants, dipole moments, relative energies, vibrational frequencies, transition state structures, and the activation energy for the isomerization between the cis- and trans-HSiOH isomers. Basis sets as large as triple zeta plus double polarization plus silicon and oxygen atom f and hydrogen atom d functions [TZ2P(f,d)] have been used with the self-consistent-field configuration interaction including all single and double excitations (CISD), and coupled cluster including all single and double substitutions (CCSD) methods, as well as CCSD with the effects of connected triple excitations added perturbatively [CCSD(T)]. Our predictions for the dipole moment components and geometry of silanone (H2SiO) were instrumental in its recent microwave spectroscopic identification (accompanying paper by Bogey and co-workers) and are in excellent agreement with the experimental results. The silanone (H2SiO) molecule is predicted to lie about 0.5 kcal mol(-1) lower in energy than the HSiOH isomers. In contrast with previous theoretical work, cis-HSiOH may be slightly more stable than trans-HSiOH. The experimental IR spectrum for the HSiOH isomer, which may have been misassigned to trans-HSiOH, is closer to that for cis-HSiOH at the TZ2P(f,d) CISD level of theory. The isomerization between cis- and trans-HSiOH. takes place along the torsional mode, and the activation energy is predicted to be 8.3 kcal mol(-1).