A novel inline method using microphotometric measuring principle is able to determine mean particle sizes of polymer blends directly during extrusion processing. This process-micro-photometer (PMP) is adapted on a special slit die at the extruder head. Refractive indices needed for particle size calculation have been measured by an inline refractometer for all the involved polymers at molten state. For investigation, blends have been made from polystyrene (PS), polypropylene (PP), polyamide (PA6), and polymethyhymethacrylate (PMMA); and the resulting blend morphologies are found to be smaller than 1 mu m in all cases. Sensor calculations show good correlation to microscopic reference measurements. Because of multiple scattering effects at larger particle number concentrations an exact particle size calculation is limited to low volume concentrations of that polymer blends because of the small blend particle sizes. (C) 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 258-262, 2007