A down#ow reactor for fast gas}solids reactions has been developed and characterised for the particular case of #uid catalytic cracking. With very short contact times and essentially plug #ow 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 modi"ed #uidised bed feeder, in which the catalyst is heated prior to being fed into the downer with a high mass #ow rate constancy (#uctuations typically lower than 5%). Cracking of cumene has been chosen as reaction system for a "rst 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 #ow 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 #ow, so that the catalyst is fully exposed to the gas mixture.