Single bubble velocity profile (velocity vs. distance) has been measured for a 1.45 mm diameter bubble to try to explain reported differences in gas holdup in two systems: F150 vs. 1-pentanol, and MIBC vs. NaCl. It was found that in F150 the bubble slowed rapidly to reach the terminal velocity stage of the profile while in 1-pentanol the bubble remained in the deceleration stage and was consequently moving faster, at almost twice the speed. The lower rise velocity in F150 means longer bubble residence time and corresponding higher gas holdup than with 1-pentanol, as reported. In the second system, the bubble in MIBC reached terminal velocity more rapidly than in NaCl which agrees with the reported higher gas holdup in MIBC compared to NaCl. An implication for gas holdup evaluation is that the value may depend on the part of the velocity profile over which it is measured. Possible exploitation of this reagent effect on bubble rise velocity in flotation is considered. (C) 2010 Elsevier B.V. All rights reserved.