Experiments were carried out in a specially designed high-density coupled circulating fluidized bed system. Fluidized catalytic cracking (FCC) particles (rho(p) = 1300 kg/m(3), d(p) = 69 mum) were used. When the solids circulation flux is 400 kg/m(2.)s, the apparent solids holdup exceeds 20% near the top of the riser A, and the volumetric solids fraction (apparent solids holdup) is larger than 5.2% in the fully developed region of the downer. Hence, a high particle suspension density covers the entire coupled CFB system. Under the high-density conditions, the primary air rate had a small influence on the solids circulation flux, while the secondary air rate had an important effect on it. The results indicate a particle acceleration region and a fully developed region were identified along the downer from the pressure gradient profiles. In the fully developed region of the downer, the volumetric solids fraction increases with increasing solids circulation flux or decreasing superficial gas velocity U-1. (C) 2004 Elsevier B.V. All rights reserved.