Experiments on maldistribution. were performed using a 289 min diameter air-sand fluidized bed with a multiorifice distributor. The minimum superficial gas velocity (U-m) required to eliminate maldistribution was measured; the results were combined with those in the literature and used to evaluate the following rules for maldistribution: (1) the pressure drop ratio method, that is, a specified ratio of distributor pressure drop to bed pressure drop, R, as recommended by Perry et al.,(1) (2) the theory of Fakhimi and Harrison,(2) based on a pressure balance for the gas passing through active and inactive gas injection points at the distributor, and (3) the theory of Yue and Kolaczkowski,(3) a modification of Fakhimi and Harrison's theory.(2) The data cover a variety of bed sizes and shapes including the industrially sized beds of Whitehead and Dent 4 who used tuyere distributors. The best agreement was found with the theory of Fakhimi and Harrison,(2) which gives a simple formula for predicting U-m if combined with reliable predictions(2,5) for the height of the entry zone, h. The average error in predicting Um this way was 18%, and the largest error was a factor of 2. The traditional method,(1) using a specified R, shows a largest error of a factor of 6 in U-m if the best value of R is used.