The purpose of this study was to assess the level of agreement between two techniques commonly used to measure exogenous carbohydrate oxidation (CHO_EXO). To accomplish this seven healthy male subjects (24 ± 3 y, 74.8 ± 2.1 kg, 62 ± 4 mL/kg/min) exercised at 50 % of their peak power for 120 min on 2 occasions. During these exercise bouts subjects ingested a solution containing either 144 g glucose (8.7 % w.v. GLU) or water (WATER). The glucose solution contained trace amounts of both [U-¹³C]-glucose and [U-¹⁴C]-glucose to allow CHO_EXO to be quantified simultaneously. The water trial was used to correct for background ¹³C enrichment. ¹³C appearance in the expired air was measured using isotope ratio mass spectrometry while ¹⁴C appearance was quantified by trapping expired CO₂ in solution (using hyamine hydroxide) and adding a scintillator before counting radioactivity. CHO_EXO measured with ¹³C-glucose (¹³C-CHO_EXO) was significantly greater than CHO_EXO measured with ¹⁴C-glucose (¹⁴C-CHO_EXO) from 30-120 min. There was a 15 ± 4 % difference between ¹³C-CHO_EXO and ¹⁴C-CHO_EXO such that the absolute difference increased with the magnitude of CHO_EXO. Further investigations suggest that the difference is not due to losses of CO₂ from the trapping solution prior to counting or an underestimation of the 'strength' of the trapping solution. Previous research suggests that the degree of isotopic fractionation is small (Kalhan et al. (1977)). Therefore, the explanation for the discrepancy in calculated exogenous CHO oxidation remains to be fully understood.