Both carbohydrate (CHO) and caffeine have been used as ergogenic aids during exercise. It has been suggested that caffeine increases intestinal glucose absorption, but there are also suggestions that it may decrease muscle glucose uptake. The purpose of the study was to investigate the effect of caffeine on exogenous CHO oxidation. In a randomized cross-over design, 8 male cyclists (27±2 y, 71.2±2.3 kg, 65.7±2.2 ml/kg/min) exercised at 64±3%VO₂max for 120 min on three occasions. During exercise subjects ingested either a 5.8% glucose solution (Glu, 48g/h), glucose with caffeine (Glu+Caf, 48 g/h + 5 mg/kg/h), or plain water (Wat). The glucose solution contained trace amounts of [U-¹³C]-glucose so that exogenous CHO oxidation could be calculated. CHO and fat oxidation were measured by indirect calorimetry, and ¹³C appearance in the expired gases was measured by CF-IRMS. Average exogenous CHO oxidation over the 90-120 min period was 26% higher (p<0.05) in Glu+Caf (0.72±0.04 g/min) compared with Glu (0.57±0.04 g/min). Total CHO oxidation rates were higher (p<0.05) in the CHO ingestion trials compared with Wat, but were highest when Glu+Caf was ingested (1.21±0.37 g/min, 1.84±0.14 g/min and 2.47±0.23 g/min for Wat, Glu and Glu+Caf respectively, p<0.05). There was also a trend (p=0.082) towards an increased endogenous CHO oxidation with Glu+Caf (1.81±0.22 g/min versus 1.27±0.13 g/min for Glu and 1.12±0.37 g/min for Wat). In conclusion, compared to glucose alone, 5 mg/kg/h of caffeine coingested with glucose increases exogenous CHO oxidation possibly as a result of an enhanced intestinal absorption.