The Co-Kneader is well known for its superior mixing performance and its exact temperature control capabilities. Therefore it is widely used in the polymer industry for the compounding of shear- and temperature sensitive materials like PVC or high filled compounds. In contrast to the considerable amount of scientific work that deals with investigation, modeling or simulation of the process behavior of single and twin screw extruders, there are only few publications about the CoKneader. Due to increased quality requirements and the trend to cost reduction by process optimization, this is increasingly becoming a problem for plant construction and processing companies. In order to address this problem, this paper presents experimental investigations of the process behavior of Co-Kneaders. Based on the experimental results a physical-mathematical model to estimate the process behavior of planetary roller extruders is in development. The basic approach is to divide the extruder in small sections with constant geometry conditions. For these sections all material parameters can be considered as constant. Then solid conveying, melting, melt transport, energy input und temperature development can be modeled separately and coupled afterwards in a simulation tool.