A numerical study was conducted for investigating the size segregation of binary mixtures in the presence of fines in bubbling gas-solid fluidized beds. The gas flow was modeled using the continuum theory and the solid phase was modeled by the discrete element method. The experimental data on the extent of segregation from literature were used to validate the simulation results of this pseudo-2D model. The effect of adding fines at different concentrations and various sizes was investigated. The results were analyzed in terms of inter-particle and fluid-particle forces as well as the coordination number of particles to comprehend the effect of fines on changing these parameters. The results suggested that adding fines increases segregation of small and large particles by lowering inter-particle interactions which results in enhanced mobility of larger particles. Furthermore, reducing the size of fines from 900 mu m to 500 mu m enhances the final extent of segregation while further decrease in size of fines diminishes the segregation. Full separation of fines from large and small particles was not achievable during the segregation process and a portion of fines remained in lower layers rather than being collected at the bed surface. (C) 2012 Elsevier B.V. All rights reserved.