Stibnite (Sb2S3) forms in the heat-exchanger units of several New Zealand binary geothermal power stations. By analysing aqueous samples collected from two representative plants, it was determined that stibnite forms at a rate of 8.7 and 15.8 kg/day at the Rotokawa and Ngawha power stations, respectively. These results were compared to theoretical predictions of stibmte solubility. It was shown that pH change is the principal cause of stibnite deposition at Rotokawa, while at Ngawha the effect of temperature decrease is more significant. Antimony was not detected in vapour-line samples, suggesting that transport is completely within the aqueous phase. (c) 2007 CNR. Published by Elsevier Ltd. All rights reserved.