This paper reports experimental investigation of mixing performances inside microfluidic liquid slugs using the Villermaux-Dushman reaction. Slug-based microfluidics offers rapid mixing by internal circulation and transport with narrow residence time distribution, making it suitable for precise reaction and mixing operations. The experimental investigation presents operation methods to form microfluidic liquid slugs having the reactant arrangements. Mixing performances of flows with and without internal circulation are compared and mixing rates for the axial and radial arrangements of reactants are also compared. Moreover, for the radial arrangement, mixing performances can be evaluated using a dimensionless number irrespective of flow rate and slug length, based on the novel method discussed in our previous paper, which proposes the modified Peclet number P-e* to estimate mixing rates and design liquid slugs to obtain desired mixing rates. These results give support to the suggestions by the previous simulation results. (c) 2007 Elsevier Ltd. All rights reserved.