A downflow reactor for fast gas-solids reactions has been developed and characterised for the particular case of fluid catalytic cracking. With very short contact times and essentially plug flow behaviour this reactor type should improve reaction selectivity, provided that the design of solids feeding device, mixing zone and gas-solids separator is appropriate. The test unit makes use of a modified fluidised bed feeder, in which the catalyst is heated prior to being fed into the downer with a high mass flow rate constancy (fluctuations typically lower than 5%). Cracking of cumene has been chosen as reaction system for a first test series. At very short contact times in the range of 60-400 ms, conversions up to 70% have been determined. Amongst others, this is the result of one of the main assets of the downer, viz. the fact that the catalyst mass flow rate is not limited like in riser reactors (choking ratio). However, experimental data indicate that high catalyst-to-oil ratios, up to 60, only result in high conversions when the solids jet entering the downer reactor can be transformed into a homogeneously dispersed two-phase flow, so that the catalyst is fully exposed to the gas mixture.