Abstract: Process and apparatus, in which a stream of liquid reactants is heated in a resonant microwave cavity operating in a TM.sub.0n0 mode and then provided with sufficient time in an annular plug flow reactor to complete the reaction results in improved time-temperature uniformity for all volume elements of the stream and are particularly suited for production of polyurethane prepolymer. A special cleaning method is provided for removing polymeric residues from the equipment with a solution of a monofunctional secondary amine in an organic solvent.