Purpose: To examine the effect of normobaric hypoxia on pulmonary oxygen uptake (VO2) and muscle oxygenation kinetics during incremental and moderate-intensity exercise in children. Methods: Eight prepubertal boys (9–11 y) performed incremental cycle tests to exhaustion in both normoxia and hypoxia (fraction of inspired O2 of 15%) followed by repeat 6-minute transitions of moderate-intensity exercise in each condition over subsequent visits. Results: Maximal oxygen uptake (VO2 max) was reduced in hypoxia compared with normoxia (1.69 [0.20] vs 1.87 [0.26] L·min−1, P = .028), although the gas exchange threshold was not altered in absolute terms (P = .33) or relative to (VO2 max (P = .78). During moderate-intensity exercise, the phase II VO2 time constant (τ) was increased in hypoxia (18 [9] vs 24 [8] s, P = .025), with deoxyhemoglobin τ unchanged (17 [8] vs 16 [6], P ≥ .28). Conclusions: In prepubertal boys, hypoxia reduced VO2max and slowed ˙VO2 phase II kinetics during moderate-intensity exercise, despite unchanged deoxyhemoglobin kinetics. These data suggest an oxygen delivery dependence of VO2max and moderate-intensity VO2 kinetics under conditions of reduced oxygen availability in prepubertal boys.