Two strains of bacteria that were known to be capable of degrading thiocyanate were isolated from the Youanmi gold mine in Western Australia. Both strains demonstrated the ability to utilise thiocyanate as their sole energy and nitrogen source. Physiological characterisation indicated that both strains had the potential to tolerate the variable conditions encountered in Youanmi tailings water. Phylogenetic analysis showed that the strains were either members of the genus Thiobacillus or Halomonas but could not be accommodated within any existing described species. Phosphate was the only additional nutrient required.Both strains were inoculated into a laboratory-scale rotating biological contactor, where degradation took place in saline, low-nutrient water. The biomass supported on the reactor surface (20 m(2)) was capable of degrading approximately 2800 mg thiocyanate L-1 to less than 1 mg L-1 at a flow rate of 30 mL min (-1) and a hydraulic retention time of 11.1 h. When this degraded water was used as the basal medium for testing the bacterial oxidation of ferrous ion and of arsenopyrite and pyrite, the rates obtained were similar to those obtained with thiocyanate-free medium. This showed that degradation was successful, that the by-products of the reaction, ammonium and sulphate ions and carbon dioxide, were not toxic to iron- and sulphide-oxidising bacteria, and that water could be recycled to a biological oxidation plant after passage through the thiocyanate-degrading reactor. (C) 2001 Elsevier Science B.V. All rights reserved.