It has been observed by many authors that for ascending bubbly flows, the gas fraction distributes radially in different manners. Depending on the flow conditions and channel characteristics, the bubbles may accumulate in the center of the pipe (core peak) or near the walls (wall peak). The radial distribution of gas fraction significantly affects the friction coefficient and the heat transfer. Therefore, it is important to be able to predict which distribution will appear under certain flow conditions. In this note, by analyzing a vast experimental data base, a criterion to identify the transition from core to wall peak is proposed. The transition occurs when both Reynolds and Weber numbers reach critical values (R-crit = 1500, W-crit = 8). This criterion is validated with our experimental results and with those found in the literature. (C) 2012 Elsevier Ltd. All rights reserved.