In terms of variation characteristics of radial voidage profiles, we had identified that all fluidization regimes from minimum fluidization to pneumatic transport prevailing in circulating fluidized bed (CFB) risers can be grossly divided into two distinctive variational regions, the dense phase continuous bubbling flow region and the dilute phase continuous clustering flow region, at a cross-sectionally averaged voidage of 0.75 [Powder Technol. 101 (1999) 91]. Succeeding that work, this article is devoted to further estimating the radial voidage profiles in two such distinctive variational regions by concerning the case of circular circulating fluidized bed risers. A new correlation was proposed for the dense phase continuous bubbling flow region, while the equation of Zhang et al. [Chem. Eng. Sci. 46 (1991) 3045] was considered applicable to the dilute phase continuous clustering flow region. Further improving the equation of Zhang et al. according to the result obtained in our last report ensured the continuity between the two correlations. In addition, the article also highlighted the formation mechanics of the two distinctive variational regions for radial voidage profiles in terms of the different gas/particle hydrodynamic behaviors in bubbling and clustering flows. (C) 2004 Elsevier B.V. All rights reserved.