The equilibrium structure and anharmonic force field of the silaformyl anion, HSiO-, have been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), with basis sets of double-through quintuple-zeta quality. The vibrational-rotational energy levels of the main isotopic species were calculated by both variational and perturbational approach. The accurate ground-state spectroscopic constants were then determined, which (we believe) can be useful in future detection of the silaformyl anion in the gas phase and/or inert-gas matrixes by means of the vibrational-rotational spectroscopy. In conjuction with the results of analogous calculations on silanone (H2SiO) and hydroxysilylene (HSiOH). the gas-phase proton affinities of the HSiO- anion are predicted to be 349.6 and 348.2 +/- I kcal/mol, respectively, at 0 K. Calculations were also performed for the SiOH- isomer, yielding the gas-phase proton affinity of this anion of 391.1 +/- I kcal/mol at 0 K. Correcting these values for the thermal energy contributions, the basicities at 298 K, DeltaG degrees, of the HSiO- anion are predicted to be 343.1 and 341.7 +/- I kcal/mol, respectively, while that of the SiOH-anion is 384.8 +/- I kcal. The former two values are substantially lower than the experimental basicity of the HSiO-anion reported in the literature.