Two types of transition to turbulent fluidization are proposed on the basis of bubble-bubble interaction. A relatively sharp transition (type I) is triggered when bubbles grow to such a size that following wakes become open and turbulent. The type II gradual transition occurs when the wake is closed at the transition point. A mechanistic model is developed to predict the type I transition and is found to be consistent with experimental results obtained in a column of 102-mm diameter and with extensive data fi om the literature. The model also successfully predicts the variation of U-c with particle properties, measurement location, and system pressure.