The hydrodynamic behavior of binary solids circulating fluidized beds was examined in terms of axial distributions of apparent solid holdup as well as of fine and coarse solid holdups. Experiments were carried out in two circulating fluidized beds of 3 m in height and 97 mm and 150 mm in diameter, using FCC catalysts with a mean diameter of 69.7 mum as the fine particles and 321-mum silica sand and 633-mum activated alumna as coarse particles. The influences of gas velocity solids circulation rate, loading ratio of coarse particles and bed diameter on the axial distributions of apparent holdup and of fine and coarse particles holdups were investigated. Comparisons were made of the axial distributions of solids holdups both in the presence and absence of coarse particles. It was found that the addition of coarse (or dense) particles results in a significant variation in both the apparent and the fine particles holdups. In addition, the nonuniformity of the axial solid holdup distribution was found to vary with increasing loading ratio of coarse particles.