Flow behaviour of fluid catalytic cracking,particles were investigated in a riser with volumetric solids concentrations of 20% and more. The air velocity and solids circulation flux had only a small influence on the solids hold-up once high-density conditions were attained. At these high-density conditions, refluxing of solids near the riser wall, commonly observed in low-density CFB risers, disappeared and was replaced by a more homogeneous flow structure. The slip velocity increased with solids hold-up and, for constant superficial gas velocity, there appears to be a unique relationship between the two. Slip factors as high as 10 were obtained in the developed region of the riser, compared to values of about 2 to 5 reported in the literature for more dilute fully developed flows in the fast fluidization flow regime. Differential pressure fluctuations increased with increasing suspension density and were not significantly affected by superficial gas velocity and height. A transition point is defined to distinguish dilute from high density conditions in the CFB riser.