The dynamics of formation of solute peaks in microfluidic systems are investigated by computer simulation. A finite-element numerical procedure is applied to analyze the diffusion- and flow-controlled concentration dispersion in a 40 μm-high rectangular flow-through channel. Two-dimensional concentration profiles are shown for channels with cross sections of large aspect ratio. The final shapes of the peaks are formed during a very short time period, ranging from a few milliseconds to about 1 s for low and high flow velocities, respectively. The observed standard half-width σ of the peaks is found to strictly follow a linear function of t1/2 over the whole time range. The extrapolated long-term peak characteristics are in perfect agreement with theoretical predictions. For comparison, theoretical results on the concentration dispersion for solute peaks in open-channel liquid-chromatography (HPLC) are re-examined and applied. © 2008 Elsevier B.V. All rights reserved.