Both ageing and physical inactivity are associated with impaired pulmonary oxygen uptake (VO2) kinetics and ultimately, exercise tolerance. However, VO2 kinetics are sensitive to exercise and nutrition intervention, and strategies that augment blood flow may speed VO2 kinetics and enhance exercise tolerance in an ageing population. Cocoa flavanols (CF) are known to improve vascular endothelial function and augment nitric oxide (NO) production, but their potential to modulate VO2 kinetics during exercise has not yet been studied. Therefore, the objective of this study was to test the hypothesis that, compared with placebo (PL), CF would speed VO2 kinetics during moderate-intensity exercise and enhance exercise tolerance in sedentary middle-aged adults. Seventeen healthy, sedentary adults (11 male, 6 female) were assigned in a randomized, double-blind, crossover design to receive daily cocoa extract (400 mg flavanols, 11.6 mg caffeine, 90 mg theobromine) or PL (0 mg flavanols, 11.6 mg caffeine, 90 mg theobromine) supplementation for 7 days. Initially, maximal VO2 (VO2 max) and the gas exchange threshold (GET) were determined. Subsequently, participants were familiarised with procedures and the time to exhaustion (TTE) trial. Following 7 days of PL/CF supplementation, participants completed a series of step-exercise tests: three 6-min bouts of constant-load cycling at 80% GET and one bout at 60%Δ to exhaustion. Pulmonary gas exchange, heart rate (HR), blood lactate and perceived exertion was measured throughout. Resting blood pressure was also recorded. After a one-week washout period, participants underwent identical tests on day 7 after a secondary PL/CF daily supplement cross over regime. No differences were found in resting systolic (PL: 128 ± 12 vs CF: 127 ± 12 mmHg) or diastolic (PL: 78 ± 7 vs CF: 78 ± 7 mmHg) blood pressure between PL and CF conditions. In response to moderate-intensity exercise, heart rate amplitude (AHR) and time constant (HRτ) were similar between PL and CF (AHR, PL: 31 ± 8 vs CF: 32 ± 8 b.min−1, P = 0.516; HRτ, PL: 53 ± 22 vs CF: 47 ± 13 s, P = 0.219). Likewise, Δ blood lactate was similar between conditions for moderate- (PL: 1.2 ± 0.9 vs CF: 1.3 ± 0.8 mM, P = 0.760) and also severe-intensity exercise (PL: 7.4 ± 2.5 vs CF: 7.1 ± 2.8 mM, P = 0.694). Compared with PL, CF reduced the fundamental time-constant (τVO2) during moderate-intensity exercise (PL: 35 ± 12 vs CF: 30 ± 7 s, P = 0.047). All other VO2 parameters (VO2b, AVO2, TDVO2, Gain and End-exercise VO2) were similar during moderate-intensity exercise between PL and CF. No difference was observed in TTE between conditions (PL: 435 ± 58 vs CF: 424 ± 47 s, P = 0.480). Dietary supplementation with CF appears to speed VO2 kinetics during moderate-intensity exercise but does not impact exercise tolerance in sedentary middle-aged adults.