['We sought to explain the unexpected failure of the inorganic copper-based biocide CuWB50 to effectively decontaminate microfibre cleaning cloths that became contaminated with Acinetobacter lwoffii.', 'CuWB50 was diluted using distilled water or tap water obtained from two different ICUs. Microtitre plate assays were used to determine the minimum inhibitory concentration (MIC) for the implicated A. lwoffii. pH and oxidation-reduction potential (ORP) tests were performed and representative water samples were chemically analysed. When diluted in distilled water, the CuWB50 MIC for A. lwoffii was 9 mg l(-1) but in tap water from each ICU it was 37 and 75 mg l(-1) at hardness levels of 246 and 296 mg CaCO(3) l(-1) respectively. CuWB50-distilled water solutions consistently had a lower pH and higher ORP than CuWB50-tap water solutions.'] Is the efficacy of the inorganic copper-based biocide CuWB50 compromised by hard water?