Non-uniform bubble growth occurs in deep fluidized beds, resulting in generation of large bubbles or even slugs. Gas tracer method, in combination with analysis of pressure fluctuations, is employed to quantify the time-averaged and transient characteristics of bubble growth with respect to the bed depth in a 300-mm ID cold flow unit. The bubble distribution, growth and its residence time in the transition from uniform to non-uniform bubble growth and finally to slugging are investigated. In addition, appreciably better conversion and selectivity in a 1200-mm ID industrial fluidized bed reactor with ammoxidation of p-chlorotoluene (PCT) is obtained by eliminating non-uniform bubble growth. (C) 2017 Elsevier Ltd. All rights reserved.