Samples of diffusion juice showing varying quality were filtrated using an ultrafiltration process in a laboratory. The process was carried out using membranes made of cellulose nitrate (CN) and polyvinyl chloride (PVC). In the diffusion juices and filtrates obtained there were assayed the contents of dry mass, sucrose, reducing sugars, and a-aminoacid nitrogen, as well as viscosity. The juice purity level achieved and the final purification effect were calculated. It was stated that, owing to the filtration with cellulose nitrate (CN) and polyvinyl chloride (PVC) membranes, the content of sucrose and reducing sugars remained unchanged. After the ultra-filtration the juices had a smaller a-aminoacid content compared with the diffusion juices, and these substances were more accurately separated when filtrated through a CN membrane than through a PCV membrane. The viscosity of juices decreased by 0.7–1.8 mPa·s after the completed filtration with CN membranes, and by 0.5–1.7 mPa×s when filtrated using PVC membranes; this parameter was close to the viscosity of pure sucrose solutions of a similar concentration value. A PVC membrane was characterized by a greater flux of permeate than a CN membrane, especially in the initial phase of the filtration. The greater the purity and lower the viscosity of the diffusion juice, the greater the permeate flux. The application of membrane filtration made it possible to purify juices of bad quality, which are very hard to purify under industrial conditions. Juices filtered through a cellulose nitrate membrane were characterized by a better quality compared with the juices filtered through a polyvinyl chloride membrane.