An ultrasound in-line monitoring system that includes a homemade probe and a fast data acquisition system was used to investigate the melting behavior of linear low density polyethylene (LLDPE) in pellet form and four polyvinyl chloride (PVC) compounds in powder form in an intermeshing counter-rotating twin-screw extruder. Ultrasound signal patterns obtained from experiments revealed various melting phenomena in Cchambers, depending on materials, processing conditions, and screw configurations. The experimental results suggest that PVC particles were suspended in the polymer melt in the melting process of most PVC compounds, while melted film was still observed in the melting process of the PVC/dimethyl phthalate (DMP) system and the PVC/polybutylene adipate (PBA) system. Based on the analysis of wave attenuation, the normalized amplitude ratio K was used to characterize the melting level and uniformity across screw channels at screw speeds of 40 and 50 rpm, respectively. (c) 2005 Society of Plastics Engineers.