Frothers can be classified (ranked) according to the gas holdup vs. concentration relationship generated when gas is sparged into a water column. One argument for using gas holdup is a correspondence with bubble size through the effect on bubble velocity: An increase in gas holdup signifies a decrease in bubble velocity associated with a decrease in size. This correspondence is examined in this communication. The method of determining bubble size is described. With MIBC as the example frother, two regions of the gas holdup vs. concentration relationship were examined: low concentration where, rather than increasing, gas holdup varies little; and high concentration where, rather than becoming constant, a steady increase in gas holdup occurs. The first was associated with bubble size not having decreased sufficiently to reduce bubble velocity. In the second case the increase is associated with a steady increase in the population of small bubbles that is not readily detected by the commonly used Sauter mean parameter. Lastly, the bubble size at equal gas holdup for a suite of frothers was examined. Rather than being the same the bubble size showed significant differences. This implies that there is a frother type (chemistry) effect, either on the velocity of the single bubble or the swarm. The evidence for such a frother effect is discussed. (c) 2007 Elsevier B.V. All rights reserved.