Abstract The particle-size distribution, the settling-velocity distribution and the concentration-weighted settling velocity are important parameters in the study of sediment dynamics. These parameters can be determined by means of an optical settling tube in which the concentration of particles is determined by means of a beam-attenuation meter and the particle size is determined by Stokes' settling. Results obtained by this method compare very well with Coulter Counter and gravimetric settling tube data. Various correction factors are discussed. The instrument is designed for eventual use in situ.