Effects on the phase holdups and overall bed voidage of gas and liquid maldistribution due to partial blockage of the distributor and due to their premixing below the distributor have been determined for three-phase fluidized beds in a 127-mm-diameter column. The experiments were carried out with 3.3-mm polymer particles and 3.7-mm glass beads (densities of 1280 and 2510 kg/m(3)), with water and air as the liquid and gas. Six different gas-liquid distributors were used, having various combinations of gas and/or liquid maldistribution, in addition to one with a very uniform distribution of both, At low gas flow, the voidage was similar for all distributor types tested, However, there were major effects of distributor geometry at higher gas velocity, especially for the lighter particles, due to circulation arising from the maldistribution caused by the gas-liquid distributor. A uniform distributor leads to a higher overall voidage than a non-uniform one unless the latter produces significantly smaller gas bubbles. The effect of non-uniformity is explained by applying the gas-perturbed liquid model (Powder Technol. 100 (1998) 113) to extend the particle circulation theory of Hiby (in: Drinkenburg (Ed.), Proceedings of the International Symposium on Fluidization, Netherlands University Press, Amsterdam, 1967, pp. 21-30) and Masliyah (AOSTRA J. Res. 5 (1989) 49).