This work addresses the enzymatic hydrolysis of beechwood xylan on a membrane reactor with tangential flow over an ultrafiltration membrane. The hydrolysis was carried out on concentration mode, with circulating substrate, and results were compared with those obtained for batch assays. The oligosaccharides in the permeate and concentrate were characterised by high-performance size exclusion chromatography (HPSEC). The HPSEC analysis of the permeates shows a high concentration of oligosaccharides with molecular weights between 6000 and 1500 Da, and a smaller concentration of short chain oligosaccharides, with molecular weights between 600 and 450 Da. This fraction of lower molecular weight is also present in the concentrate, and may be due to the high concentration factors attained. The fluxes declined from 43 to 33 kg/(h.m(2)), and the percentage of total sugars retained in the reactor was 52% within a period up to 4 h of reaction.