Some results obtained in a photocatalytic membrane reactor, to be used for degradation of toxic organic species dissolved in water, are reported. The catalyst, TiO2 P25 Degussa, was immobilised by means of a flat sheet polymeric membrane and 4-Nitrophenol (4-NP) was used as a model molecule to evaluate the reactor performance. A preliminary investigation of the stability, under UV irradiation, of some eligible polymeric membranes was carried out by using scanning electron microscopy (SEM), optical microscopy (OM), determinations of water permeation flux (WPF) and total organic carbon (TOC). These tests showed that commercial membranes made of fluoride + PP (FS 50 PP-Dow), polysulphone + PP (GR 51 PP-TechSep) and polyacrylonitrile (PAN-TechSep) seemed to be quite stable to UV light over a 24 h period of irradiation. Immobilisation of TiO2 onto membranes by ultrafiltrating TiO2 suspensions showed an optimal layer density slightly >2.04 mg TiO2 per square cm of membrane surface area. Results obtained from membrane reactor studies indicated that the observed initial rate constants for the degradation of 4-NP were almost independent on the amount of TiO2 employed over the range 0.76-4.08 mg/cm(2) A 50% weight degradation of 4-NP after 5 h of irradiation in the presence of air was obtained. Instead, an almost complete degradation of 4-NP was observed in the presence of TiO2 suspended in the solution and pure oxygen. The permeate deriving from the membrane photoreactor was clear and 4-NP concentration was approximately equal to that found in the retentate. The possibility of the continuous reuse of the photocatalyst and the continuous separation of products from the reaction medium give some advantages over traditional approaches, (C)2000 Elsevier Science B.V, All rights reserved.