Complex geometries encountered in many applications of gas-solids flow need special treatment in most legacy multiphase flow solvers with Cartesian numerical grid. This paper briefly outlines the implementation of a cut cell technique in the open-source multiphase flow solver-MFIX for accurate representation of complex geometries. Specifically, applications of the Cartesian cut cell method to different gas-solids fluidization systems including a small scale bubbling fluidized bed with submerged tube bundle and a complete pilot-scale circulating fluidized bed will be presented. In addition to qualitative predictions on the general flow behaviors inside each system, quantitative comparison with the available experimental data will be presented. Furthermore, some results on extending the current cut-cell technique to Lagrangian-Eulerian simulations will be presented. (C) 2012 Elsevier B.V. All rights reserved.