Local solids concentration fluctuations were measured in a long downer reactor (0.1 m ID, 9.3 m tall) using an optical fiber probe. Axial flow development and radial flow dynamics were analyzed using both statistical and chaos methods. Core, transition and annulus regions were identified and each region showed different flow behavior. Cross-sectional averaged chaos parameters were correlated to cross-sectional averaged solids holdup to develop relationships between non-linear flow dynamics and operating conditions. Crown Copyright (C) 2007 Published by Elsevier B.V. All rights reserved.