In a recent paper, Matsuyama et al. [2007. operation of microfluidic liquid slug formation and slug design for kinetics measurement. Chemical Engineering Science 62, 5133-5136] reported an experimental investigation of mixing performances inside microfluidic liquid slugs using the Villermaux-Dushman reaction. We comment on the veracity of the experimental results. Because 12 produced in the aqueous phase could be extracted into the dimethicone phase, an error is introduced in the mixing performance characterization. To overcome this problem, the amount Of 12 in the dimethicone phase should also be measured. (C) 2008 Elsevier Ltd. All rights reserved.