Taylor-Aris dispersion in narrow-bore capillaries is used to measure the diffusion coefficient of colloidal particles in aqueous suspension. The method is shown to yield accurate results for particles up to about 0.3 mu m in diameter; the measurement time for larger particles is prohibitively long and impractical. For hydrophobic particles, interactions with the capillary walls can introduce error into the interpretation of the data. The measurements also suggest that buoyancy-driven particle motion can introduce error. Consequently, a method similar to capillary hydrodynamic fractionation was developed to establish when these factors were of negligible effect. The results constitute an order-and-a-half improvement in the sensitivity of the technique, which has been recently shown to work for nanometer-sized proteins. The data suggest that, when matched with the appropriate theory, dispersion in capillaries may be a useful probe of colloidal and gravitational interaction potentials.