Numerical simulations were performed to study the mixing dynamics of two miscible liquids within a T-shaped micromixer, comparing the case where the two inlet fluids are both water (W-W) with that where they consist of water and ethanol (W-E). We showed that at smaller Reynolds number, Re < 100, there is no vortex formation for either cases; therefore, mixing water and ethanol is slightly easier than mixing water with water because the residence time of the fluid occupying the interfacial region in the W-E case is larger than that of the W-W case. On the other hand, at larger Reynolds numbers, mixing water and ethanol may take considerably longer, as the onset of engulfment is retarded and occurs at larger Reynolds number. All these behaviors are due to the fact that, as a W-E mixture has a viscosity that is up to almost three times larger than that of water, at the confluence of the T-mixer the two streams are separated by a viscous layer of a W-E mixture, that hampers vortex formation and retards mixing. (c) 2013 Elsevier Ltd. All rights reserved.