The time lag has long been known to contain information regarding the transport and reaction properties of a membrane. When more one than of these properties contributes to the time lag, extra information is required to resolve the relevant contributions. It is shown that a ratio of areas derived from the difference between the cumulative flux curve and its asymptote may provide such information. The effects of various membrane reaction parameters on this ratio are studied for a particular model of permeation where reversible binding and irreversible removal of both free and bound permeant occur. (c) 2004 Elsevier B.V. All rights reserved.