Few routes to well-defined 3D silicone structures exist because of their susceptibility to depolymerization/metathesis in the presence of acids or bases. The Lewis acid B(C6F5)(3) can be employed to condense hydrosilanes with alkoxysilanes, producing siloxanes and alkanes (R3SiH+R'OSiR ''(3) -> R3SiOSiR ''(3) + R'H). We demonstrate that balancing the steric demands at both the hydrosilane and alkoxysilanes, and the careful control of reaction conditions, permits clean condensation reactions to occur in the absence of competing metathesis processes. The resulting linear or highly branched siloxane compounds can be rapidly and easily assembled into explicit, complex 3D silicone structures in high yield.