Dense-dilute phase structure is a common feature of gas-solids flow in circulating fluidized bed. This paper presents an experimental study of dense-dilute phase characteristics in a high-density downer (5.6 m tall, 80 turn ID), in which solids holdup as high as 15% has been achieved. The quantitative characteristics of dense-dilute phase, such as time fraction, duration time, frequency, have been investigated to show the effects of operating conditions, radial position and axial elevation. Preliminary comparison of the phase structure in a high-density downer to that in a high-density riser has been carried out in this work. (c) 2005 Elsevier B.V. All rights reserved.