Gas barrier plastic films and structures capable of removing a permeating solute via irreversible chemical reactions in the polymer matrix are investigated in applications of active packaging for controlling contained environments. To significantly reduce permeation rates through such active barriers, high barrier reactivity is required. When the reactive species take the form of fine particulates sparingly dispersed in the matrix, the reactivity of a composite membrane is affected by a lower frequency of collisions of dissolved gas molecules with the distributed reactive particles. The mean size of the particles and their volume fraction in the matrix are identified as the critical factors affecting the overall reactivity of uniformly filled composite barriers. A two-scale coarse-grained model of reaction-diffusion in a matrix is developed by introducing a unit cell approach to estimate the reaction rate upon collision with the particles and by choosing the unit length of diffusive molecular displacement equal to the mean diameter of the reactive particles to describe diffusion across the barrier and statistics of molecule-particle collisions. The corresponding method for evaluating the barrier reactivity is described. The effects of the system parameters on the effective gas transport rates across the reactive composite barriers including polymer nanocomposite barriers with reactive additives are quantified.