The screening of catalysts, substrates or conditions in the early stages of bioprocess development requires an enormous number of experiments and is a tedious, expensive and time-consuming task. Currently available screening systems can only be operated in batch or fed-batch mode, which can lead to severe misinterpretations of screening results. For example, catalysts that are inhibited by substrates or accumulating products will be excluded from further investigations in the early stages of process development despite the fact that they might be superior to other candidates in a different operational mode. Important and advantageous properties such as turnover stability can also be overshadowed by product inhibition. The aim of this study was to develop a novel screening system that enables continuous feeding of substrates and continuous removal of products. A prototype based on the membrane reactor concept was designed and operated for a model reaction, the hydrolysis of cellulose.