The effect of microchannel aspect ratio (channel depth/channel width) on residence time distributions and the axial dispersion coefficient have been investigated for Newtonian and shear-thinning non-Newtonian flow using computational fluid dynamics. The results reveal that for a fixed cross-sectional area and throughput, there is a narrowing of the residence time distribution as the aspect ratio decreases. This is quantified by an axial dispersion coefficient that increases rapidly for aspect ratios less than 0.3 and then tends towards an asymptote as the aspect ratio goes to 1. The results also show that the axial dispersion coefficient is related linearly to the Reynolds numbei when either the aspect ratio or the mean fluid velocity is varied. However, the fluid Peclet number is a linear function of the Reynolds number only when the aspect ratio (and therefore hydraulic diameter) is varied. Globally, the results indicate that microchannels should be designed with low aspect ratios (<= 0.3) for reduced axial dispersion. (C) 2008 Elsevier B.V. All rights reserved.