An experimental research involving tracer studies, carried out in four geometrically similar models of contact chambers and operated under an equal detention time criterion, is presented. The scale effect on dispersion characteristics is investigated to determine how closely the performance of a prototype can be predicted from a laboratory study on scaled-down models. A number of parameters and indices, including dispersion number, are considered in the investigation. It is observed from the output-generated curves that the coefficient of dispersion is influenced by scale. However, the normal variance and the dispersion number are, to a large extent, independent of scale, at least for longitudinal basins larger than 5.6 cm (2.2 in.) width and for all practical detention times.Key words: longitudinal dispersion, dispersion number, model study, scale.