Tests with five different neat polymers, two different blends and one composite were conducted on a Haake Rheomix 3000 and Rheomix 600 internal laboratory mixers with two different types of rotors. The heat transfer coefficient per unit area was determined from the temperature and torque values recorded by each equipment. In all cases, dispersion of the experimental data was significant. Results showed that the heat transfer coefficient in internal mixers is higher than in common single screw extruders and similar to the values found for corrotational twin screw extruders. Data indicates that heat transfer coefficient in internal mixers is independent of the internal mixer capacity, the type of rotor used, the processing temperature and the type of neat polymer processed. In contrast, it is dependent on the nominal speed of the rotors and the polymer for incompatible polymer blends and for polymer matrix composites. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.