Abstract Gravity separation of bidisperse suspensions containing particle species lighter and heavier than the suspending fluid in a vertical tube has been studied both theoretically and experimentally. The suspension was relatively dilute having a total solids volume concentration of no more than 16%. In this dilute range, lateral segregation of light and heavy particles into clusters and formation of fingering flow structure do not occur. The settling velocities of both particle species are retarded. Five models published by Lockett and Al-Habbooby, Mirza and Richardson, Masliyah, Selim et al. and Patwardhan and Tien, were used to predict the settling velocities of the heavy and the light particle species in bidisperse suspensions. The last three of these models give good predictions for the experimental data obtained in this study.