AbstractMaximum spoutable bed depths for mixed particle‐size beds of alundum, glass and polystyrene have been found experimentally to reach a peak at a critical particle Reynolds number of about 70. Using Becker's criterion that Vm f = Vmf and the correlations of Smith‐Reddy for V, and of Wen‐Yu for Vmf, the maximum spoutable bed depths Hmax were theoretically predicted to reach a maximum at a critical NRem = 68. The experimental maximum spoutable bed depts in a 6‐in. I. D. column fitted with a 60° cone were in good agreement with theoretical predictions.