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Thin polymeric barrier layers are often interposed between sensitive surfaces and other polymeric layers in which corrosive species slowly evolve as by‐products of curing or decomposition. The corrosive species considered here are chemically reactive gases. Reducing the number of reactive species that can diffuse to sensitive surfaces ameliorates problems with corrosion, debonding, and the loss of mechanical strength or dielectric breakdown resistance. As some of the diffusing, reactive species are chemically immobilized in the barrier, the polymer’s diffusion coefficient is reduced. We write the resulting nonlinear diffusion equation in a form having special cases which are linear, facilitating the required analysis of multilayered systems. In the case of high doses of reactive gas impinging on one face of a thick slab of polymer, we calculate concentration profiles that agree well with the available experimental data, supporting the validity of the linearization procedure. In the case of low doses of corrosive species evolving over long periods of time in one layer of a two‐layer system, we calculate typical concentration profiles and compute the barrier effectiveness of the second layer in protecting an adjacent surface. The barrier effectiveness is seen to increase faster with a decreasing barrier diffusion coefficient than with an increasing diffusant‐immobilization rate in the barrier. Graphical results for the barrier effectiveness are given as a function of the barrier thickness and diffusion coefficient.