ABSTRACTAn intermittent strong vegetable/cabbage odour was noted on raw water abstracted from the River Dee (NW England). The source of the odour was traced to an industrial discharge from an animal feed manufacturer to a tributary of the River Dee. Analysis of water samples by CLSA GC‐MS revealed the likely causes of the odour to be dimethyl disulphide and dimethyl trisulphide. 2‐hydroxy‐4‐(methylthio)butanoic acid (HMB), a hydroxy methionine analogue growth supplement used by the feed manufacturer was a likely precursor to the odour problem. Studies conducted on the behaviour of HMB in untreated water showed it to be rapidly degraded. Degradation products included dimethyl disulphide, dimethyl trisulphide and methyl(methylthio)methyl disulphide. The odour of water samples incubated with HMB changed from that of potato to cabbage/sprout, indicating that breakdown of this compound was responsible for the vegetable odour on untreated rive