Using an optical detector we followed the transient state of blends and composites, including a reactive blending during extrusion. The detection system is composed of a slit-die with transparent windows fixed at the extruder exit, an optical arrangement with a W incandescent light microbulb with fixed luminescence, and a CdS photocell. As the tracer passes though the light path, it absorbs and backscatters part of the light, reducing the total transmitted light intensity. This is followed by changes in the voltage induced by the photocell to an electric circuit We calibrated the response of the photocell at room temperature using a set of various films with a second phase dispersed, and obtained a logarithmic relationship. The tracers were particulate (phthalocyanine, TiO2) and polymeric (PS, PA6) phases that absorb and scatter light, producing a residence time distribution (RTD) curvelike trace. Measurements were taken from a twin-screw extruder Werner-Pfleiderer ZSK 30 equipped with K-Tron gravimetric feeders operating at various screw configurations and speeds, and feeding rates. The transient state of PP/PA6 blends can be easily detected optically and recorded using one of the components (either PP or PA6) added as a pulse in a steady-state flow of the other component With the simultaneous addition of a compatibilizer (polypropylene grafted with acrylic acid (PP-g-AA)) with the PA6, the intensity of the detector signal is substantially increased as a result of the PA6/PP-g-AA reaction. Quantitative off-line infrared spectroscopy of the total amide group corroborated the in-line measurements. These observations suggest that an in-line optical detector may be a fast and simple way to study the flow behavior of blends and composites, including reactive processing. (C) 2004 Society of Plastics Engineers.