Dissemination of an analyte under the laminar flows plays a major role in measuring and assessment of biological fluids such as sample preparation in the context of microfluidic systems. Due to the development of manufacturing technology in the Lab-on-a-chip devices, the production of rectangular microchannels with finite aspect ratios and micron and submicron sizes has been provided by which the effect of electrokinetic phenomena on concentration distribution will be magnified in these systems. Since the recent researches in this field have overlooked such effects, the present work will be conducted analytically to study the effect of electric double layer on cross stream diffusion of the analyte in the combined electroosmotic and pressure driven flows. Three flow scenarios, the favorable, adverse and zero pressure gradients are analyzed. The results demonstrate that the width of the diffusion region near the top and bottom walls of the microchannel becomes broader with the increase in the Debye length. Also, the results of the scaling analysis reveal the decrease in mixing intensity with increasing the Péclet number based on Helmholtz-Smoluchowski velocity and dimensionless Debye–Hückel parameter. As well, the average scaling exponent of this criterion is a descending function with respect to the thickness of the electric double layer.