The use of microspheres and nanospheres of poly(d,l-lactide-co-glycolide) (PLG) as a controlled-release drug delivery system has been the subject of great interest for at least two decades within the field of pharmaceuticals. Salts of zinc and other divalent cations are sometimes co-encapsulated in PLG particles to control the pH or to stabilize encapsulated proteins or peptides. Zinc salts are known to affect pore formation and other processes that may lead to the release of an encapsulated drug. In this study the effect of encapsulated zinc acetate on protein diffusion through PLG films was investigated. PLG films, with and without encapsulated zinc acetate, were degraded in Hepes buffer for different periods of time. The films were subsequently subjected to various kinds of analyses: diffusion properties (using a diffusion cell), porosity (using scanning electron microscopy) and thickness (using light microscopy and an image-analysis program). Encapsulated zinc acetate had a considerable effect and increased the diffusion coefficient of lysozyme through PLG films degraded for 18 days or longer. Films containing zinc acetate became porous, while those without zinc acetate only developed cavities on the surface. Zinc salts may thus be used as release-modifying agents. This effect should be considered when using zinc salts as protein stabilizers or pH neutralizers.