A method to design mixing in microfluidic slugs using a modified Peclet number, Pe* = U(s)d(s)(2)/lD, has been reported by the authors, but it was limited to mixing at constant diffusivity D. This paper reports an improved method to quantitatively determine the effect of D on a relation between Pe* and mixing rates. Computational fluid dynamics (CFD) simulations were used for the investigation. We introduce D into the mixing rate term in the relation between Pe* and mixing rates, and found that (mixing rate x d(s)(2)/D) becomes a function of only Pe*. Thus, slug mixing can be designed using the new dimensionless number, (mixing rate x d(s)(2)/D), and Pe*. This allows us to use mixing rate data at any value of D to estimate mixing rates at another value of D. Though Pe* includes effects of D, 1, as, and Us, effects of initial arrangements of reactants inside a slug and slug cross-sectional shapes are not considered. Thus, the relations between (mixing rate x 4,21D) and Pe* (referred as Pe* correlation) are quantitatively determined to cover the effects of these parameters. Furthermore, we used the Pe* correlation to show theoretically that channel contraction is an effective microfluidic operation to enhance mixing in slugs. (c) 2006 Elsevier Ltd. All rights reserved.