The interdiffusion of two materials used to fabricate polymeric gradient refractive index (GRIN) lenses was examined by varying contact time during multilayer films co-extrusion composed of alternating poly(methylmethacrylate) (PMMA) and poly(styrene-co-acrylonitrile) with 17 mol% acrylonitrile (SAN17). The model applied successfully described their interdiffusion, and a reasonable mutual diffusion coefficient of 7.0 x 10(-13) m(2)/s was determined. Atomic force microscopy confirmed good agreement between modeled profiles and actual layer structure, and optical properties were investigated. Films with contact times of <= 160 s exhibited multiple refractive indices, while films with longer contact times, showed single refractive indices that followed an additive line. Though a single additive value requires layers <= a quarter-wavelength of light, these films exhibit such behavior with thicknesses 5x greater than expected, though layer resolution is still present. Using the model profiles, it was determined that only a 1% reduction in material purity is required to reduce the effective layer thickness. (C) 2012 Elsevier Ltd. All rights reserved.