The equilibrium structure and vibrational harmonic frequencies of the silanol anion, SiH3O-, have been determined from ab initio calculations using the coupled-cluster method, CCSD(T), with basis sets of double-through (partly) quintuple-zeta quality. In conjuction with the results of analogous calculations on silanol, the gas-phase proton affinity of the silanol anion is predicted to be 356.2 +/- 2 kcal/mol at 0 K. Correcting this value for thermal energy contributions, the gas-phase acidity of silanol is predicted to be 350.7 +/- 2 kcal/mol at 298 K, in good agreement with the experimental data.