High flux gas-solids circulating fluidized bed downer reactors have unique characteristics especially suitable for very fast reactions. Detailed hydrodynamics of such reactors are experimentally studied using FCC particles at various superficial gas velocities (1-7 m/s) under high flux conditions up to 700 kg/m(2)s for the first time. Results show that although the radial distribution of solids holdup is somewhat less uniform under very high flux conditions in the downers, it is still much more uniform compared to riser reactors. Radial profiles of solids holdup, particle velocity and solids flux are significantly affected by the operating conditions. Particle velocity distribution is characterized by a relatively flat core and an annulus, where the particle velocity slightly increases towards the wall under low flux. In very high flux downer reactors, the shape of the local particle velocity becomes parabolic. It is also found that relationships between local solids holdup and particle velocity are different in the downers compared to the risers due to their different flow characteristics. Compared to the riser reactor, downer has a self-adjusting mechanism to perform with the nearly "ideal" plug flow nature. (C) 2016 Elsevier B.V. All rights reserved.