The present invention concerns a matrix reactor for reactions in which low-molecular and high-molecular reactants are involved wherein the reactor space V.sub.R contains a matrix and comprises a reaction compartment and a supply compartment, wherein the matrix is composed of a porous material which can take up low-molecular reactants and wherein the exclusion volume V.sub.0 of the matrix is available as the reaction compartment and the matrix volume V.sub.M of the matrix is available as the supply compartment. The matrix reactor can be used for enzymatic processes, in particular for a coupled in vitro transcription/translation reaction to produce proteins.