Capillary electrophoresis (CE) is an efficient separation method in analytical chemistry. It exploits the difference in electrophoretic migration velocities between charged molecular species in aqueous or diluted polymer solution when an external electric field is applied to achieve separation. Despite the standard assumption that electrophoretic data obtained from pulse-loaded molecular species should have Gaussian peak shapes, experimentally observed peaks are frequently distorted or highly asymmetric. Interaction of charged species with the wall of the capillary is the primary source for serious band broadening and peak tailing. This paper reports a mathematical model for the peak profiles in capillary electrophoresis, taking adsorption on capillary wall into account. The model is based on the advection–diffusion equation, Langmuir second order kinetic equation and appropriate boundary conditions. It is applied to simulate the gel electrophoretic separation of the 11 fragment Φ X174-Hae III double stranded DNA ladders in a polymeric microchip. By using the migration velocities and diffusivities from the measurement, and properly selecting two fitting parameters, namely adsorption and desorption coefficients, the simulated peak shapes show remarkable similarity with the experimental electrophoretic results. The effect of adsorption and desorption coefficients are also investigated and the result shows that adsorption of analytes from the main analyte zone and desorption of these analytes appear to be the reasons of peak tailing, with the latter being the major cause.