Disclosed is a process for carrying out a cyclization reaction, a polymerization reaction, an enzymatic reaction showing substrate inhibition, an enzymatic reaction showing product inhibition, a reaction showing precipitation of the substrate or of a reactant, the process comprising the steps of a) diluting a fresh substrate with solvent to form a diluted substrate-solvent mixture, and supplying this mixture to a reactor, b) causing the reaction medium in the reactor to react, c) discharging reaction mixture comprising reaction product, solvent, and substrate that has not reacted, to a first filtration membrane which is permeable to the solvent and impermeable to the substrate and to the catalyst or at least one of the reactants, d) returning solvent from the permeate side of the first membrane to dilute the fresh substrate, and e) returning retentate comprising substrate which has not reacted, from the first filtration membrane to the reactor.