An apparatus is described for dialyzing aqueous solutions and simultaneously concentrating them to a thick syrup, or even to dryness. The fluid is placed in collodion bags with reservoir bulbs open to atmospheric pressure. The bags are surrounded by flowing cold water, under negative pressure, and are subjected of a rocking motion. Over a liter of fluid can be concentrated to a few cubic centimeters in 3 to 4 hours. It is kept cold and sterile during the process.