A light-scattering photometer with optical microscope for in-line studies during polymer processing is described. The instrument utilizes a commercial twin-screw extruding device that feeds molten polymer into a narrow slit die equipped with sapphire optical windows. The flow rate through the die is controlled via the feed rate, screw rate, and a by-pass valve. The optics for light scattering and microscopy can be switched back and forth via simple translation of a rail-mounted optical platform without realignment, allowing efficient in situ morphological studies in both real and reciprocal space. Extruded polystyrene/polyethylene blends and melts are used to demonstrate the performance and versatility of the instrument.