Adsorption in rectangular and square channels is attracting great interest from various fields of research. In particular, in large scale applications the main interest is in the prediction of the performance of monoliths composed of several individual channels, while, in the field of microengineering, particular interest is in the separation performance of individual channels for the development of small-scale analytical devices, that is, the lab on a chip. The design of all these units is based on the accurate representation of the breakthrough dynamics and the performance of the unit can be represented conveniently by the height equivalent to a theoretical plate (HETP). General simplified equations for the calculation of the HETP are derived for rectangular channels of arbitrary aspect ratio. The dispersion in the solid phase is corrected to take into account the effect of the adsorptive capacity in the four corners of the solid phase. A corrected thickness of the walls of the channels is predicted and shown to yield the exact HETP by comparing the analytical solution to the full 3-D numerical solution. Numerical simulations are presented for representative gas and liquid systems. (c) 2005 American Institute of Chemical Engineers.