A perturbation (or pulse) method was used to investigate the melting of a polystyrene/polypropylene (PS/PP) blend in a 40-mm twin-screw extruder (TSE). A sliding-barrel technique was used to visualize the melting processes, map the temperature and pressure profiles along the channel, and obtain the residence time distribution (RTD) at different locations in the extruder. Pressure, pulse, and visualization results were used to determine where melting occurred. Three runs with ratios of the flow rate/screw speed (Q/N) varying from 5.0 to 11.3 g/revolution were studied. It was found that the melting of the PS/PP blend in the TSE had three distinct regions. Most of the melting occurred in a narrow transition region (∼50 mm) from the partially filled region to the fully filled region. The location of the transition region was found using four different techniques: visualization, pressure, temperature, and combined pulse/RTD methods. High-speed video of the extrusion processes shows that the solid polymer pellets melted through an "erosion" mechanism. Mechanical energy consumption for melting can be obtained through this perturbation method.