The combination of two or more polymers within co-extruded multi-layered products provides tailored properties, which cannot be achieved by using pure plastics alone. To guarantee excellent product quality, adhesion between the individual layers is of utmost importance. The interpenetration of macromolecules across the interface, the so-called interdiffusion, is the prevalent bonding mechanism for compatible polymer pairs and the thickness of the resulting interdiffusion layer is a direct measure for the overall adhesion strength. The occurrences at polymer-polymer interfaces under shear load are still a topic of scientific debate. While the orientation of the macromolecules is thought to slow down interdiffusion, an overall increase in interdiffusion layer thickness is observed. There are numerous experimental approaches to determine the position of the interface and its thickness. Confocal Raman microscopy has proven to be an outstanding method for this purpose, as it is a label-free technique that enables imaging with a resolution below 500 nm. In this work, the applicability of confocal Raman microscopy to characterize interdiffusion is demonstrated for the compatible model system poly(methyl methacrylate) and poly(styrene-co-acrylonitrile). The concentration profiles across the diffuse interphase are determined via spectroscopic imaging, which allows the measurement of the interdiffusion layer thickness with high reproducibility. In conducting static diffusion experiments, the influence of contact time and temperature on the formation of the interdiffusion layer is studied. To investigate the additional effect of shear load, co-extruded sheets were prepared and analyzed. With these findings, an enhanced understanding of phenomena at co-extruded polymer-polymer interfaces can be gained, which aids significantly in the development of novel multi-layered products.