The classical premixed isothermal flow reactor continues to be of interest in combustion and chemical engineering applications. For many of these problems, such as the combustion of toxics, the degree of conversion in a finite-length reactor is a critical consideration. Assuming global kinetics, we focus on the completeness of combustion for arbitrary reaction orders and stoichiometry, for both steady and nonsteady inflow conditions. For the case of steady inflow, it is shown that the deficient species may decay either algebraically or exponentially, or the reaction may go to completion, depending on the reaction order of the deficient component. Although this feature is preserved for nonsteady inflow conditions, sinusoidal (zero-mean) fluctuations in the inflow velocities and concentrations result in a nonzero time-average perturbation in the deficient component at any given downstream spatial location. Consequently, it is shown that when time-dependent perturbations in the inflow conditions are present, a longer reactor is generally required to achieve the same average degree of conversion as for the case of the steady-stale reactor.