A dynamic model has been developed for the size fractionation process of gas-filled microspheres by flotation. The aim of this dynamic model is to control the fractionation process in an Nycomed Imaging AS Infoson(TM) production plant. A general population balance is established for the dispersed phase that is later simplified for the specific particle suspension and process equipment used. This dynamic model has been tested against experimental data and shows a good correlation between the predicted and measured values. This paper describes both the physical phenomena and the modeling method.