Garncarek’s quantitative index of inhomogeneity has been used to compare the effect of distributor type on the degree of uniformity of solids mixing in a gas-fluidized bed assessed by following a tracer particle using the technique of Positron Emission Particle Tracking. The bed was of a sand of 475 mu m diameter fluidized at 2U(mf) and contained within a column of 150 mm diameter. A porous and perforated plate distributor are compared. The index shows that tracer locations are generally more homogeneously distributed throughout the vertical section of the bed with the porous plate distributor, Behaviour varies with height and horizontal sections of the bed show that there is little difference at the bottom, close to the distributor, where the bubbles are numerous and small. The porous plate gives a more uniform degree of mixing in the middle of the bed (presumably the greater degree of inhomogeneity with the perforated plate reflects the influence of more stable bubble streams rising from the apertures in it). There is less difference towards the top of the bed where bubbles have grown to fill much of the cross section of the bed irrespective of distributor type.