The present paper proposes a detailed comparison of mixing efficiency of different mixers that have been characterized by the Villermaux/Dushman test reaction. Considering simple relations of mixing in laminar flow, it is shown how to obtain the theoretical mixing time and how to relate it with operating parameters as the Reynolds number of the flow and the specific power dissipation per mass unit of fluid. The comparison of the experimental and of the theoretical mixing times indicates that only a few percents of the total mechanical power transmitted to the fluid is effective for mixing. (C) 2009 Elsevier Ltd. All rights reserved.