The microstructure and chain diffusion behavior at a rubbery/glassy polymer interface (PS/ PPO) were studied using the depth-resolved technique of secondary ion mass spectroscopy (SIMS). Unlike the typical rubbery-rubbery interface, the microstructure of the miscible glassy-rubbery interface demonstrated a very sharp symmetric profile with a thickness around 25 nm before annealing. As chain diffusion took place, the glassy-rubbery interface moved toward the PPO region, the interface broadened asymmetrically, and the chains diffused across the interface showing both the Fickean and the case II characteristics simultaneously. For the purpose of analysis, the interfacial layer can be divided into a rubbery region and a glassy region according to the local T g calculated from the local polymer concentration using the Flory-Fox equation. The case II characteristic dominated on the glassy side of the interface, where chain mass flow required plasticization of the glassy polymers by the neighboring rubbery chains. The Fickean characteristic, on the other hand, prevailed on the rubbery side of the interface, where polymers at both sides of the interface were rubbery chains. The effect of temperature on the interdiffusion was in good agreement with that predicted by the WLF equation. The average velocity of the interface decreased with the PS molecular weight following a negative power law. The power of the dependence, however, was different for the two molecular weight regimes above and below the critical molecular weight of PS (M c ) 38K). The mutual diffusion coefficient and the tracer diffusion coefficients were determined from the SIMS data. The results were in good agreement with the slow theory predictions.