In circulating fluidized bed (CFB)-reactors, large cyclones are integrated into the external solids recycle loop to separate efficiently highly loaded gas-solids suspensions leaving the reactor. The separation performances of recycle cyclones (d(a), = 0.63 m; h(z), = 2.19 and 2.91 m, respectively) with and without a vortex finder were studied under ambient conditions. A variety of solids fractions (40 < d(50.3) < 300 mu m; corresponding Ar-number 5 < Ar < 2000) was used covering the whole regime of advantageous CFB-operation. A sufficient and continuous acceleration of the particularly segregated gas-solids suspension in the cyclone entrance duct originates in necessary high solids momentum fluxes at the cyclone entrance performing excellent separation efficiencies (eta(ges) > 99.97% for fine particles with d(50.3) = 40 mu m) due to the formation of a single, relatively compact solids strand. A proper solids strand separation allows for the elimination of the traditional vortex finder without any disadvantages for the separation efficiencies. Results of measured local solids concentrations and solids velocities confirm significant solids segregation in the cyclone entrance area as well as remarkably high gas-solids slip velocities. Furthermore, the experimental investigations showed opportunities for an adequate design of overall CFB-solids circuits. Only an interference-free flow of the externally recycled solids guarantees lowest solid losses with the clean cyclone off-gas.