Whey protein isolate (WPI) is widely used as a protein ingredient in foods, but undergoes thermal treatment before and during application to make the final product. The aim of the present study was to determine the temperature dependency of formation of volatiles that potentially act as off-flavours in reconstituted WPI solutions (3%, w/v) heated for different temperature–time combinations (50–90 °C for 5–10 min). Differences in orthonasal aroma were evaluated by triangle tests by a sensory panel. The panel could distinguish between samples heated for 10 min at 60 °C and 70 °C. Analysis by headspace gas chromatography-mass spectrometry (GC-MS) showed that heating the samples at 90 °C for 10 min significantly increased the concentration of dimethyl disulphide and dimethyl trisulphide compared with heating at 80 °C for 10 min or below, which suggests that sulphur-derived odour could be decreased by decreasing the heating temperature by 10 °C.