Introduction: Glucose co-ingestion has been shown to enhance splanchnic extraction and metabolism of ingested galactose at rest, but effects during exercise are unknown. The aim of the present study was to examine whether combined ingestion of galactose and glucose during exercise enhances exogenous galactose oxidation. Methods: 14 endurance-trained participants (age, 27 [5] years; V̇O2peak, 58.1 [7.0] ml·kg-1·min-1) performed stationary cycle ergometry for 150 min at 50% peak power output while ingesting beverages providing carbohydrates at rates of 0.4 g.min-1 galactose (GAL), 0.8 g.min-1 glucose (GLU) and on two occasions 0.8 g.min-1 total galactose-glucose (GAL+GLU; 1:1 ratio). Single-monosaccharide 13C-labelling (*) was used to enable calculation of individual (GAL, GLU, GAL*+GLU, GAL+GLU*) and combined (GAL*+GLU*, COMBINE) exogenous-monosaccharide oxidation between 60-150 min of exercise. The research was approved by the Science, Technology, Engineering and Mathematics Ethics Committee, University of Birmingham, Birmingham, UK in accordance with the Declaration of Helsinki. Results: Plasma galactose concentrations with GAL+GLU (0.4 mmol.L; 95%CL 0.1, 0.6) were lower (contrast: 0.5 mmol.L; 95%CL 0.2, 0.8; P<0.0001) than when GAL alone (0.9 mmol.L; 95%CL 0.7, 1.2) was ingested during exercise. Exogenous carbohydrate oxidation with GAL alone (0.31 g·min-1; 95%CL 0.28, 0.35) was marginally reduced (contrast: 0.05 g·min-1; 95%CL -0.09, 0.0; P=0.01) when combined with glucose (GAL*+GLU 0.27 g·min-1; 0.24, 0.30). Total combined exogenous-carbohydrate oxidation (COMBINE: 0.57 g·min-1; 0.49, 0.64) was similar (contrast: 0.02 g·min-1; 95%CL -0.05, 0.09; P=0.63) when compared with isoenergetic GLU (0.55 g·min-1; 0.52, 0.58). Conclusion: glucose co-ingestion did not enhance exogenous galactose oxidation during exercise. When combined, isoenergetic galactose-glucose ingestion elicited similar total exogenous-carbohydrate oxidation to glucose suggesting that galactose-glucose blends are a valid and equivalent alternative for glucose as an exogenous-carbohydrate source during exercise.