While the choice of the location of the liquid distributor in a liquid-solid circulating fluidized bed is application dependent, three different methods exist for operating the riser in the circulating fluidization regime. Experiments were conducted to study the effect of the location of the liquid distributor, method of operation, liquid viscosity, and solids inventory on the axial solids holdup distribution in the circulating fluidized bed regime. The results show that the location of the liquid distributor has a significant effect on axial solids holdup, and the axial solids holdups obtained by the three methods of operation are different under identical conditions. The variation of the axial solids holdup distribution with liquid viscosity with a solid inventory of 0.15 m was different from that with both 0.25 and 0.35 m solid inventories. There is significant interaction of the solids inventory and liquid viscosity on the axial solids holdup distribution. © 2012 American Chemical Society.