A comparison is made of drop diameters produced by a disk and an open type six-blade impeller having the same impeller diameter and width and rotated at the same speed. Drop size measurements in situ at 200, 250, 300, 350, 400 and 450 rpm and at hold-up fractions 0.01, 0.025, 0.05 and 0.10 showed that always the Sauter mean drop diameters produced by the open style impeller were 6-82% lager than the ones produced by 0 the disk impeller. Plots of In d(32) versus In N and In d(max), versus In N gave straight lines. Plots of d(32) versus d(max) gave relationships not quite linear. (c) 2005 Elsevier B.V. All rights reserved.