Birch sap (Betula pendula Roth, and B. pubescens Ehrh.) was concentrated by reverse osmosis in a pilot scale recirculating system using ZF-99 tubular membranes. Sap feed temperatures varying from 30-70°C and pressures from 4-5 MPa were used when concentrating the sap from about 0.7oBrix to 13oBrix. Optimal conditions considering both processing capacity and product quality were 50°C and 5 MPa. Retentions of glucose, fructose, sucrose, and malic acid were practically 100% when using the ZF-99 membrane, whereas with the cellulose acetate membrane the retentions fell to between 92 and 99% at 30°C and 4 MPa. The pH of the original sap varied between 6.0 and 6.6 and dropped during the run by 0.2-0.3 pH units.