We propose that variations in fat and carbohydrate (CHO) oxidation by working muscle alter O₂ uptake (O₂) kinetics. This hypothesis provides two predictions: 1) the kinetics should comprise two exponential components, one fast and the other slow, and 2) their contribution should change with variations in fat and CHO oxidation, as predicted by steady-state respiratory exchange ratio (RER). The purpose of this study was to test these predictions by evaluating the O₂ kinetic model: O₂(t) = _R + _F{1  exp[(t  TD)/_F]} + _C{1  exp[(t  TD)/_C]} for short-term, mild leg cycling in 38 women and 44 men, where O₂(t) describes the time course, _R is resting O₂, t is time after onset of exercise, TD is time delay, _F and _F are asymptote and time constant, respectively, for the fast (fat) oxidative term, and _C and _C are the corresponding parameters for the slow (CHO) oxidative term. We found that 1) this biexponential model accurately described the O₂ kinetics over a wide range of RERs, 2) the contribution of the fast (_F, fat) component was inversely related to RER, whereas the slow (_C, CHO) component was positively related to RER, and 3) this assignment of the fast and slow terms accurately predicted steady-state respiratory quotient and CO₂ output. Therefore, the kinetic model can quantify the dynamics of fat and CHO oxidation over the first 5-10 min of mild exercise in young adult men and women.