Abstract Drinking or heated water can have a wide range of disturbing odours which have many various causes. A well known example is sulphane (hydrogen sulfide) odour, reminiscent of rotten eggs, which naturally occurs in some underground waters. Less known and hitherto unexplained is this odour arising from hot water in water heaters. As a response to complaints by consumers we investigated 14 sites and thanks to experimental work in one of these recognised a complex mechanism at work. For production of sulphane to occur in a water heater the following conditions must be met: presence of sulphate-reducing bacteria, presence of sulphates, reduction properties of water indicated by a very low nitrate content (highest value was 10 mg/L, mostly <2 mg/L), and presence of sacrificial magnesium anode. We identified four possible remedial measures, some of which are less effective or only short term, or have an undesirable side-effect (corrosion). The most effective measure was replacement of the magnesium anode for an aluminium or zinc anode, which retains the anticorrosion protection of the heater.