Variational and diffusion Monte Carlo calculations are carried out on the molecules SiHn (n = 1-4), Si-2, Si2H6, and Si3H8. The anion SiH3- is also considered. Core electrons are eliminated through the use of an ab initio effective core potential. The atomization energies and various bond energies are compared with experiment and other calculations. The atomization energies of SiH-SiH4 are in good agreement with experiment and with the best other calculations. The calculated electron affinity of SiH3 is within 0.02 eV of the measured value. The calculated atomization energies of the silanes SiH4, Si2H6, and Si3H8 are more consistent with the original heats of formation of Gunn and Green [J. Phys. Chem. 65, 779 (1961); 68, 946 (1964)], without the subsequent corrections [M. W. Chase, Jr., C. A. Davies, J. R. Downey, Jr., D. J. Frurip, R. A. McDonald, and A. N. Syverud, J. Phys. Chem. Ref. Data 14, Suppl. 1 (1985)] made on the assumption that the final state of Si is amorphous.