This research is focused on understanding the effect of miscibility on interdiffusion phenomena in bilayers of polystyrene (PS) and the statistically random copolymer, poly (styrene-r-4-bromostyrene) (PBS). Miscibility in such systems can be quantified by N*chi, where N* is an effective degree of polymerization and chi the Flory-Huggins interaction parameter. In the PS/PBS system, miscibility decreases as N *chi increases. The extent of interdiffusion at a bilayer interface is ultimately determined by the miscibility of the system. A description of the effect of miscibility on interdiffusion phenomena, interfacial width, phase behavior, and fracture energy and a relationship between these properties has not been elucidated, thus limiting optimal design of polymer interfaces for technologically relevant applications. This work highlights the effect of miscibility on interdiffusion dynamics and interfacial behavior, employing both experimental and modeling tools.;Rutherford backscattering spectroscopy (RBS) was used to quantify interdiffusion in the PS/PBS bilayers. PBS volume fraction versus depth profiles were obtained from the evolution of the bromine peak in the RBS spectra of these bilayers as a function of annealing time. From these profiles, mutual diffusion coefficients were calculated. In miscible PS/PBS systems, interdiffusion of the bilayers results in a single layer of constant composition. In bilayers with PS and PBS having a disparity in N, the interface is observed to move toward the faster diffusing (lower N) component, in agreement with the fast-mode theory of mobility. In partially miscible PS/PBS layers, interdiffusion is observed to occur until binodal conditions that agree with predicted phase diagrams are reached.;Interfacial widths (wi) in PS/PBS bilayers were measured with X-ray reflectivity (XR) and are in qualitative agreement with the results of RBS. It was observed in miscible systems that features corresponding to a bilayer configuration disappeared after short diffusion times, indicating complete interdiffusion, while partially miscible and immiscible systems retained bilayers features for even the longest annealing times, indicating interdiffusion to binodal conditions or minimal interdiffusion, respectively. Implications of the miscibility dependence of the mutual diffusion coefficient and interfacial width are discussed both in the context of theoretical arguments as well as for strengthening partially miscible polymer interfaces.