Pneumatic conveying and fluidized bed systems require designers to predict possible flow regimes of the two-phase (solids-gas) flow under specific operating conditions. Although several studies have suggested various flow regime diagrams for this purpose, the effect of the pipe diameter and solid concentration on transition velocities remain open issues. The main purpose of this work is to design a generalized flow regime diagram for vertical pneumatic conveying and fluidized bed systems. We analyze numerous experimental and theoretical studies in order to understand the influence of the various parameters including particle and gas properties, pipe diameter and particle concentration. According to our empirical analysis, which is based on tens of materials, hundreds of experiments and different experimental methods we find that the influence of the column diameter on the transition velocities is in the range of the experimental error and therefore may be neglected. Additionally, we find that the four Geldart classification groups explain the appearance of different fluidization and pneumatic conveying states. Based on an empirical and theoretical analysis it is shown that the generalized flow regime diagram for vertical systems can be presented by the relationship between the Archimedes and the Reynolds numbers modified by the solid flow rate or the volumetric concentration of circulating solids and the gas void fraction. (C) 2010 Elsevier B.V. All rights reserved.