Mixed plastic waste streams are to date present in nearly all societies. Depending on the source of the plastic waste stream, the complexity and difficulty to separate and recycle the waste stream differs. In this paper, the concept of statistical entropy is used to quantify the separation complexity of mixed plastic waste streams. To this end, the recently proposed multilevel statistical entropy analysis method is extended by adding a multiproduct system level. Furthermore, an overview is presented of the research questions that can be addressed by different statistical entropy definitions. The proposed extended method is applied to a plastic packaging waste case study in Belgium for which the data are available in the literature. The results indicate that the method based on statistical entropy allows analyzing the separation complexity of real-life mixed plastic waste streams. More specifically, the multilayer films contribute the most to the separation complexity of the studied plastic packaging waste stream. In addition, it is illustrated how the method can be used to identify key contributors to the separation complexity of mixed plastic waste streams and to evaluate measures to reduce the separation complexity of mixed plastic waste streams.