The band dispersion phenomena in capillary zone electrophoresis (CZE) using untreated and surface-treated open tubular and gel-filled capillaries were experimentally evaluated, with emphasis on small capillary diameters (10-100 microns). Laser-induced fluorescence detection was used for high-sensitivity detection of the isoindoles originated from model amino acids. The plots of plate height vs electric field strength were generated for different column radii and compared with a theoretical model for CZE. In addition to the diffusion-controlled band dispersion in the relatively low electric field range, adsorptive interactions between a solute and the capillary wall may play a certain role in band-broadening. The sorption-desorption kinetics become important with increasing electric field strength. Thermal effects appear to contribute little to band-broadening in relatively small capillaries (less than 50-microns i.d.) within normal operating voltages (less than 30 kV), but could become significant in capillaries with larger bores (greater than 75-microns i.d.). With gel-filled capillaries of small diameters (less than 50-microns i.d.), diffusion processes can be minimized. In addition, thermal effects do not appear critical in such columns at reasonable voltages.