Propolis is a natural product, rich in biologically active compounds. In the recent years the product is extensively used in food and medical applications. This investigation reports results on the extraction of these compounds with an ethanol-water mixture and sequential concentration of the extract by nanofiltration, using two membranes: Starmem (TM) 122 (polyimide) and Duramem (TM) 200 (modified polyimide). The content of flavones, flavonols, flavanones, dihydroflavonols as well as total phenolic substances was determined spectrophotometrically in the feeds and permeates of dead-end nanofiltration experiments. Rejections of over 95% were observed and extract concentration of over three times was achieved with the Duramem (TM) 200 membrane. The membranes behaviour during the process was characterized by environmental scanning electron microscopy, revealing better stability and higher permeability of the modified polyimide membrane. The established conditions for extract concentration could facilitate cross-flow nanofiltration experiments regarding a technology for medicine or nutritional additive production. They could also be utilized in propolis chemistry investigations for a more reliable identification of some individual compounds. (C) 2009 Elsevier B.V. All rights reserved.