Polycarbonates are modified, nowadays, with various kinds of fillers from the micro to the nano range in order to improve properties like the electrical conductivity, for example. Rheological measurements are often performed to support the characterisation of the compounds. In this communication it is demonstrated that the crystallisation of polycarbonates in the molten state has to be taken into account for a comprehensive assessment of rheological experiments. This is particularly evident in the case of filled materials as particles can act as nucleating agents. The crystallisation behaviour of neat and filled polycarbonates is investigated by differential scanning calorimetry and its influence on rheological properties presented by dynamic-mechanical and creep experiments. From the results, the role of crystallisation processes becomes evident for a reliable interpretation of theological investigations on polycarbonates and their modelling. Comparative measurements on the non-crystallisable polymethylmethacrylate underline the importance of a thorough discussion of the crystallinity in the case of polycarbonates. (C) 2011 Elsevier Ltd. All rights reserved.