A light microscope was coupled to the flat sheet die of a laboratory extruder allowing on-line morphology measurements during the extrusion of polymer blends. Polystyrene (PS)/polymethylmethacrylate (PMMA) blends were studied in the concentration range of 5-20 wt% PMMA. image analysis of the on-line photographs allows a fast and reliable determination of the particle size distribution devoid of systematic errors that are immanent in the evaluation by transmission electron microscopy (TEM). At higher concentrations the particle dimensions are underestimated by TEM even though the dimensions are corrected for the finite thickness of the ultrathin microtome sections.