A new method for determining the effective rate constant of diffusion-controlled oxygen scavenging reactions in reactive polymer membranes with finite reactive capacity is proposed. The method is based on rapid homogeneous scavenger activation throughout the membrane and measurement of transient oxygen flux through the membrane with fixed boundary conditions according to standardized testing procedures. After the steady state flux is established across the initially passive membrane the reactive species are activated throughout the membrane thickness by an external source. The transient permeability tests are separately performed on two activated reactive films with preselected different thicknesses, scavenger loadings, and/or scavenger activities, and the minimum transient fluxes through the films are compared. The ratio of these fluxes is related to the reaction rate constant using the analytical results for the effective flux through the catalytic reactive membrane. To account for noninstantaneous and potentially inhomogeneous scavenger activation, the asymptotic behavior of the effective flux growth around the activation time is analyzed in two limiting cases. On the basis of the derived asymptotic expressions, the correction procedures for adjusting the measured minimum fluxes are proposed. The method validity is supported by numerical simulations that demonstrated excellent agreement between the analytical and numerical results for a variety of scavenger activation mechanisms.