Two commercial nanofiltration (NF) membranes (FilmTec NF-45 and Desal-5 DK) and two new NF-1 membranes made by BPT (Bio Pure Technology) for the purpose of a European Union funded research project (RENOMEM) were tested under extreme acidic conditions. The polyethersulphone (PES) ultrafiltration (UF) supports used for casting the BPT-NF-1 membranes were also tested under similar conditions. The 006 and 0 15 UF supports were found to be stable in 5% nitric acid at 20 and 80 degreesC for 4 and 3 months, respectively. Both supports (006 and 015) showed a significant reduction in flux after immersion in sulphuric acid at both temperatures. The BPT-NF-1 membranes showed excellent resistance to 20% sulphuric acid for up to 4 months at 20degreesC but were attacked by the nitric acid solution. The resistance of the two commercial membranes in 20% sulphuric acid at 20degreesC was generally lower than that of the BPT-NF-1 membranes. The NF-45 membrane was slightly more stable in 5% nitric acid at 20degreesC. Degradation of the membrane occurred only after 2 months while both the Desal-5 DK and BPT-NF-1 membranes degraded during the first month. At the higher temperature of 80degreesC in 5% nitric acid all membranes degraded in the first month. The cause of membrane degradation was attributed to oxidation of the thin NF selective skin layer in nitric acid and to acid-catalysed hydrolysis of this layer in sulphuric acid. Knowing the cause of membrane degradation is a step forward in developing a better and more stable nanotiltration membrane. (C) 2004 Elsevier B.V. All rights reserved.