The ergogenic benefits of carbohydrate (CHO) ingestion in cycling and running have been widely reported. Studies directly comparing the effect of CHO ingestion on CHO oxidation rates in cycling and running suggest no difference in exogenous CHO oxidation between these exercise modes. Potential differences in endogenous fuel use between cycling and running when ingesting CHO may lead to a greater benefit in cycling rather than running time trial (TT) performance. Direct comparisons of the effect of CHO ingestion during endurance exercise on subsequent cycling or running TT performance are limited. This study tested the hypothesis that ingesting CHO during 120 min of constant intensity exercise would benefit subsequent TT performance more in cycling than running. Methods: In a randomised, placebo-controlled, double blind crossover trial, 10 male triathletes (VO2max cycle 51.65±5.53, run 59.07±6.14 mL.kg-1.min-1) completed 4 separate exercise trials. Each trial consisted of cycling or running at 70% of mode-specific VO2max for 120-min whilst ingesting either a CHO drink (2:1 glucose: fructose, 90g.h-1) or an equal volume of taste-matched 0% CHO placebo (PLA) (750mL.h-1). After 10-min recovery, participants completed a TT in the same mode of exercise, running 6km or cycling 16km. The CHO drink was enriched with uniformly labelled 13C glucose and 13C fructose isotopes to quantify exogenous and endogenous CHO oxidation. Due to limitations of the tracer methods used in the first hour, only data for the final 60-min of exercise are presented. Total CHO and fat oxidation rates were calculated from expired air using stoichiometric equations. Data were analysed using ANOVA, values are means±SD. Results: From 60-120 min of exercise mean fat oxidation rates did not differ between exercise modes (PLA cycle 0.52±0.17, run 0.56±0.22g.min-1 p=0.86; CHO cycle 0.37±0.12 run 0.40±0.17g.min-1, p=0.82). CHO ingestion reduced fat oxidation within cycling (p<0.01) and running trials (p=0.02) compared to PLA. Mean total CHO oxidation rates did not differ between trials (PLA cycle 2.52±0.74, run 2.21±0.46g.min-1, p=0.23, CHO cycle 2.95±0.40, run 2.77±0.55g.min-1, p=0.47). Mean exogenous CHO oxidation did not differ with CHO ingestion (cycle 0.87±0.22 run 0.75±0.31g.min-1, p=0.31). CHO ingestion reduced endogenous CHO oxidation in cycling (PLA 2.52±0.74, CHO 2.10±0.41g.min-1, p=0.03) but not running (PLA 2.21±0.46, CHO 2.00±0.61g.min-1, p=0.44). TT performance improved 10% in cycling and 3% in running after CHO ingestion (p<0.01). TT performance improvement was greater in cycling than running (p=0.04, ES=1.45). Conclusions: This study demonstrates a greater improvement in cycling versus running TT performance after CHO ingestion. This may relate to the greater sparing of endogenous CHO in cycling versus running during the previous exercise period when CHO is ingested.