Segregation of solids occurs when solid particles of varying sizes and/or densities are mixed. The size and/or density differences result in heavier and/or smaller particles being concentrated toward the centre of the solid bed in a rolling drum to form a segregated core. Segregation has important energy efficiency implications in the operation of industrial rotating kilns. In this paper, a model is developed which allows the prediction of the transverse segregation of granular material in a rolling drum. The model was derived from new experiments employing image analysis. The independent variables in this work were the particle size ratio and the operational parameters of the drum and thus the model could be easily and validly applied to practical situations. The ability of the model to predict the final steady state segregation configuration was tested against independent data and good agreement was observed.