The molecular parameters of methanol, CH3OH, and its silicon analogue silanol, SiH3OH, have been determined in large-scale ab initio calculations using the coupled-cluster method, CCSD(T), and basis sets of double through (partly) quintuple-zeta quality. The properties studied included the equilibrium structure, spectroscopic constants, potential energy functions for internal rotation, and harmonic force fields of both molecules. The effects of core-electron correlation and of small-amplitude vibrations on the calculated molecular parameters were investigated. The molecular parameters determined for methanol are found to be in good agreement with the experimental data. The accuracy of the theoretical predictions for silanol can be assessed.