The aim of the present study was to select a suitable natural organic substrate as a potential carbon source for use in a denitrification permeable reactive barrier (PRB). A number of seven organic substrates were first tested in batch tests. The materials attained varying degrees of success at promoting denitrification. Some of the organic substrates performed very well, achieving complete nitrate removal (>98%), while others were considered unsuitable for a variety of reasons, including: insufficient nitrate or nitrogen removal, excessive release of leachable nitrogen from the substrate or excessive reduction of nitrate to ammonium rather than removing it as gaseous N-2. The top performing substrate in terms of denitrification extent (>98%) and rate (0.067 mg NO3--N d m(-3) d(-1) g(sub)(-1)) was then selected for two bench-scale column experiments in all attempt to simulate the PRB. The inlet concentration was 50 mg d m(-3) NO3--N and the columns operated at two different flow rates: 0.3 cm(3) min (-1) (Column 1) and 1.1 cm(3) min(-1) (Column 2). The two columns showed different general patterns, making it clear that the flow rate was a key factor at the nitrate removal. Nitrate was completely removed (>96%) by the passage through Column 1, while only partially removed in Column 2 (66%). The results indicated that the selected organic substrate (Softwood) was applicable for further use as a filling material for a PRB. (C) 2008 Elsevier Ltd. All rights reserved.