A reactor vessel for carrying out solid phase synthesis, particularly the solid phase synthesis of oligonucleotides, peptides, and combinatorial chemistry is provided. The reactor vessel comprises two or more orifices spaced from one another through which liquids within the vessel can be discharged and has a mounting enabling the vessel to be rotated about an axis so that each orifice can be moved into and out of a discharge position. Each orifice is adapted for closure by removable filtration means which are secured to the vessel to filter liquids discharging through the orifice when the orifice is in the discharge position. Processes for replacing the filtration means and for solid phase synthesis employing the use of an agitator are provided.