Measuring mass mixing in batch reactors is of great interest to prevent yield losses during scale-up of reactions. In this work, we present a novel tool to accomplish this: the heat pulse method. This is a thermal-based technique consisting of a local heat pulse, applied electrically or by a hot liquid injection, during 10 s at a power of 5-15 W and subsequent measurement of temperature increase at locations of interest. The 95% mixing time from corrected and smoothed temperature profile characterizes heat mixing. A heat mixing model identifies the contributions of thermal conduction and convection and hereby relates local heat and mass mixing in a 800 mL in a 1 L batch reactor with a 45 degrees 4 blade downward pitched turbine. The heat pulse method is applicable on different reactors, solvent independent and non-destructive. Experiments are repeatable and mixing at reactive circumstances can be mimicked. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.