The real-time measurement of the morphology of immiscible polymer blends based on polypropylene (PP) and polyamide 6 (PA6) was performed during melt blending in a twin-screw extruder. The disperse phase particle size and concentration were obtained in-line with an optical device placed at the die exit. To validate the response of the optical device, its detector was calibrated in advance in a bench using ceramic particles of well-controlled size dispersed in water. The optical device was able to measure the changes in particle size and concentration and the data enabled the calculation of the particle extinction cross section. Melt blending experiments in the extruder were performed in a transient mode, where a small amount of the material was added as a pulse to the main melt flow. The pulses containing pellets of the PA6 and polypropylene grafted with acrylic acid (PP-g-AA) were added to the PP flow stream in different amounts. The detector's response increased with increasing concentrations in the PA6 and PP-g-AA. The disperse phase particle size decreased concomitantly because of the compatibilizing effect of the PP-g-AA on the PP/PA6 immiscible blend. That observation confirmed that the detector's response was directly related to the disperse phase particle size. POLYM. ENG. SCI., 48:806–814, 2008.