The positive effects of carbohydrate (CHO) supplementation on endurance exercise are well documented but the placebo (PLA c ) effect can make the ergogenic qualities of substances more difficult to determine. Therefore, this study tested the effect of double blind ingestion of PLA c and CHO c in capsules versus known capsule (CHO k ) ingestion on prolonged exercise heat stress. Nine well trained male volunteers (mean±S.D.: 23±3 years; 62.4±6.5kg and 65.8±5.2mLkg −1 min −1 peak oxygen consumption) exercised at 60% of maximum power output until volitional exhaustion (TTE) in the three different conditions. Capsules were ingested with 252±39mL of water. Blood glucose in CHO c and CHO k was similar but higher (p<0.05) than PLA c from 45min to end of exercise. There were no differences in TTE between PLA c (125.2±37.1min) or CHO c (138.8±47.0min) or between CHO c and CHO k (155.8±54.2min). Time to volitional exhaustion was different between PLA c and CHO k (p<0.05). Increased TTE resulted when participants and researchers knew the capsule content, but not in the double blind condition. The difference could be related to a combined effect of CHO ingestion and knowledge of what was ingested possibly acting as a potent psychological motivator.