An experimental setup containing a sliding online optical device is used to monitor in real-time the melting process of a commercial polypropylene in a corotating intermeshing twin-screw extruder. Turbidity and birefringence are measured at several axial locations upstream and along the first restrictive zone of the screw, where melting develops. The experiments are performed using different set barrel temperatures, extruder feed rates, and screw speeds, to generate distinct flow histories and, accordingly, changes in the onset and rate of melting of the polymer. The local flow conditions are characterized in terms of residence time distribution and data equivalent to axial pressure profiles. Turbidity and birefringence are sensitive to changes in the operating conditions providing a coherent description of melting. The onset of melting seems to take place in partially filled conveying elements, and then melting develops quickly as the latter become fully filled, and is completed well before flow through the kneading block.