The disilaketenyl (HSiSiO) radical, an isovalent isomer of the ketenyl (HCCO) radical, has been investigated theoretically using ab initio electronic structure theory. For the two lowest-lying electronic states ((X) over tilde (2)A " and (A) over tilde (2)A') of HSiSiO, total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities were predicted at the self-consistent-field (SCF) and configuration interaction with single and double excitations (CISD) levels of theory with a wide range of basis sets. At the CISD optimized geometries coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)] energies were also determined. The ground and first excited electronic states of HSiSiO were predicted to be transplanar bent structures, while the linear 1 (2)Pi state was found to be a saddle point with two imaginary vibrational frequencies. The (X) over tilde (2)A " and (A) over tilde (2)A' states of HSiSiO are more distorted from linearity and more polar than the corresponding states of HCCO. In particular the HSiSiO ground state is predicted to have a peculiarly acute HSiSi bond angle of only 75 degrees, almost suggesting an Si-Si bridging hydrogen. At the CCSD(T) level of theory with the largest basis set, Dunning's cc-pVQZ, the first excited state was predicted to lie 36.3 kcal/mol (1.57 eV, 12 700 cm(-1)) classically above the ground state. With the same method the barriers to linearity were determined to be 45.2 kcal/mol (1.96 eV, 15 800 cm(-1)) for the ground state and 8.9 kcal/mol (0.39 eV, 3100 cm(-1)) for the first excited state, respectively. Due to their large dipole moments and relatively large vibrational infrared (IR) intensities, the two lowest-lying electronic states of HSiSiO may be suitable for IR spectroscopic studies, and the ground state for microwave spectroscopic investigations.