First results of an experimental study of the fluid catalytic cracking (FCC) process in a new type of reactor, the downflow gas-solids reactor or 'downer', are presented. A bench-scale experimental set-up has been constructed, the downer with an inner diameter of 25 mm. and a length of 800 mm being the central part of it. As a first step in the development and characterisation of this reactor, catalytic cracking of cumene has been chosen as a model reaction. In a temperature range of 743-823 K, catalyst-to-oil (CTO) ratios up to 25 and residence times of 60-350 ins, conversions were between 10 and 70%. Comparison to modelling results, on the basis of a plug flow model, showed that the measured conversions were lower than expected on the basis of the calculations. The higher the solids mass flux, the greater the difference between the two, indicating that incomplete break-up of the solids stream entering the downer - with particle clusters negatively affecting transfer processes to and from the individual particles - might be the cause for these reduced conversions.