The lag time of a barrier film can be dramatically increased by the addition of sacrificial scavengers that consume solute before crossing the barrier. This increase in lag time is independent of the scavengers' reaction rate. This reaction rate does affect the amount of solute that crosses the membrane before steady state, a premature leakage that may render the barrier unsuitable regardless of its putative lag time and permeability. A third parameter is needed to characterize the barrier's transient behavior. Guided by experiments, we offer two candidate parameters for quantifying these changes. Correlations of these parameters are determined numerically and compared with experimental results. (C) 2005 American Institute of Chemical Engineers.