An expression is derived for the plate height attributed to an analyte band transported with uniform flow through a narrow straight microchannel having arbitrary cross-section geometry. The derived expression represents the plate height as a function of the retention factor, parameters describing the adsorption dynamics, and bulk and interfacial diffusion coefficients of the analyte. Additionally, the plate height depends on a dimensionless geometrical factor, for which an expression is obtained to be valid for arbitrary cross-section geometry. Considering different examples (circle, parallel planes, annulus, arbitrary circumscribed polygon, rectangle and ellipse), it is shown that cross-section geometry strongly affects the band broadening. It is discussed how the plate height depends on the system parameters. (c) 2005 Elsevier Ltd. All rights reserved.