In this study, the dimensions of over six thousand flocs were analyzed to quantitatively and comparatively investigate the effects of several experimental variables on the growth rate of aluminum (Al) and ferric (Fe) hydroxide flocs. Results show that Fe hydroxide flocs have faster growth rate than Al hydroxide flocs; and the average size of the former is larger than that of the latter. Increasing the concentration of the bivalent sulfate ion (SO42-). initial turbidity, or slow mixing rate, was able to increase the growth rate of both kinds of flocs. On the other hand, steady floc sizes were found to decrease with the increase in SO42- concentration, initial turbidity, or shear rate. Fe hydroxide flocs are more prone to be influenced by the changes in the variables than Al hydroxide flocs. While the steady floc sizes became smaller when initial turbidity or slow mixing speed increased, the roundness and smoothness of flocs were found to increase, indicating that higher initial turbidity or larger slow mixing rate produces flocs with more regular and round shape. Furthermore, at a fixed shear rate, Fe hydroxide flocs are stronger than Al hydroxide flocs. However, Fe hydroxide floc sizes are much easier to decrease with the increase in slow mixing intensity. (C) 2009 Elsevier B.V. All rights reserved.