Both gas and solids mixing and dispersion in circulating fluidized beds have been studied using steady and unsteady state tracer techniques. Most studies, however, have been conducted in CFB risers with relatively low solids circulation rates. In recent years, the high density and high solids flux CFB riser has received increasing attention due to its applications in catalytic cracking of crude oil and potential applications in catalytic reactions requiring high catalyst-to-reactant loading ratios. In view of the significantly different flow structure in high-density and high solid flux risers where annulus solids downward flow is no longer present as revealed in some recent studies, this paper analyzed the similarity and differences of gas and solids mixing behaviour between low density and high density CFB risers based on the data recently collected in our lab and data reported in the literature. It is shown that there exists a clear transition of both gas and solids axial mixing behaviour when the flow structure changes from low density to high density operating conditions, in correspondence to the disappearance of solids downward flow near the wall region. In the low density CFB risers with solids flux lower than ~250 kg/m2s, both gas and solids axial dispersion and backmixing increased with increasing solids flux and solids concentration. Beyond the transition to the dense suspension upflow regime, however, both gas and solids axial dispersions tend to decrease with increasing solids flux, indicating the development of gas and solids flow toward the plug flow resulting from the disappearance of solids downflow in the near wall region. Radial dispersion of gas and solids, on the other hand, showed a continuous decrease over a wide range of solids fluxes within both the low density and high density flow regions, indicating a reduced lateral exchange of gas and particles with increasing solids fluxes.