An investigation has been made of fluidised systems consisting of glass beads fluidised with air. The object of the investigation was to study the spread of residence times of solids in such systems. The principal variables for investigation were:- (i) Aspect Ratio:- The length to diameter ratio of the fluidised bed. (ii) The rate of fluidising gas (Air Rate). (iii) The flow rate of solids through the system. (iv) Holdback - a parameter of mixing. (v) Segregation - a second parameter of mixing. Regression equations were produced from two factorial experiments and an attempt to correlate the results by a diffusional hypothesis was made. The expected trends were confirmed and the investigation, being one of the first of its type, opened the field for several further interesting lines of research.