Oxasilacycles are prepared by the intramolecular condensation of silanes bearing Si-bonded hydrogen and ethylenically unsaturated radicals in the presence of a triorganoborane, aminoborane complex, or phosphine-borane complex as a hydrosilylation catalyst. Spirocyclic oxasilacycles, and alkoxy-substituted oxaspirocycles with quaternary substitution of a C atom adjacent to oxygen may be prepared.