One two-phase bubble column and two three-phase slurry reactors have been experimentally characterised with special emphasis on bubble size distribution, liquid circulation and solids movement. The measurements in the two phase bubble column were based on a five point conductivity probe method and on a steady state heat tracer technique For the solids movement the CARPT technique was used. By numerical simulations using a two fluid model new models for bubble size distribution and solids presence have been tested. The new bubble size model is found to improve the size distribution predictions compared to prior models, but is still not satisfactory. Temperature profiles were well predicted by the model. For the solids movement, two circulation cells were found experimentally in the reactors tested. This has also been verified by numerical simulations. The formation of secondary cell structures of magnitude approximately equal to the column diameter, are indicated by the experimentally determined Reynolds stress patterns.