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 × d s 2 / D ) becomes a function of only Pe * . Thus, slug mixing can be designed using the new dimensionless number, (mixing rate × 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, l, d s , and U s , effects of initial arrangements of reactants inside a slug and slug cross-sectional shapes are not considered. Thus, the relations between (mixing rate × d s 2 / D ) 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.