To establish guidelines for using small silicon-containing molecules to model silicon surfaces, a series of substituted silanes have been studied using post-Hartree-Fock and density functional theories. The two theories differ in their detailed predictions, but similar trends are found with both methods. Si-Si and Si-H bond lengths vary slightly among the compounds studied. Mulliken charges on the substituted silicon atom are altered significantly, but Mulliken charges on the other atoms remain unaffected. The Si-Si bond energy decreases by about 0.5 kcal/mol with each silyl group that replaces hydrogen. Force constants change by a few percent. The most dramatic effect of substitution is that the energy of Si-H bonds at the substituted Si decrease by about 3 kcal/mol with each successive replacement of hydrogen by silyl groups. Of the properties calculated, only the Si-Si bond strength correlates with substituent electronegativity. The effects of model structure on surface ionization potentials have also been determined for comparison to earlier work. In general, substitution at a surface site alters some model properties, but more distant substitutions have little effect.