A liquid junction potential (UP) is generated at the interface of the two electrolyte solutions of different ionic concentrations. It can be applied to generate electricity and it can also be applied to passive control of the trajectory of the charged micro- and nanoparticles in micro total analysis systems. In this article, an analytical model is provided to predict the UP and the associated electrostatic field generated by the two contacting electrolyte layers in a slit-like channel, for the case of the simple 1:1 electrolyte system. The one-dimensional Nernst-Planck equation is analyzed to investigate the temporal evolution of the concentration distribution. As a result, comprehensive analytical formulas for the UP, the electric field, and the charge density are obtained. The UP is obtained by introducing the concentrations at the boundary surfaces to Planck's equation written for the finite domain. The analytical result for the UP is compared with the existing experimental result, which shows a reasonable agreement. Of particular interest is the influence of the initial concentration ratio and the thickness ratio of the two electrolyte layers. It is shown that there exist limiting profiles in the temporal evolution of the UP, with respect to the variation of the concentration ratio and the thickness ratio, respectively. The implication of the electric field produced by the charge separation is discussed concerning the behavior of the charged particles in micro total analysis systems. (C) 2003 Elsevier B.V. All rights reserved.