An apparatus is provided for conducting solid phase oligomer synthesis. The apparatus includes a reaction vessel in which a solid phase support is contained. The reaction vessel has a top opening through which gases and solvents can be delivered by way of a series of conduits and valves. The reaction vessel is interconnected through a bottom opening therein and through a series of conduits and valves with a vessel or series of vessels containing a reagent or series of reagents, respectively, required for the synthetic reaction. The reagent vessel serves a both a source of reagent delivered to the reaction vessel and as a repository for unused reagent returned thereto from the reaction vessel. Reagent delivery and mixing is gas-driven using an associated source of an inert gas.