An improved method combining mass balance and image analysis was developed in this study to determine the critical flux during crossflow microfiltration. The proposed method gave a better representation of the physical deposition processes on the membrane surface, and the results obtained were more accurate than the previous methods based on the observation technique alone. In addition, by studying the fractional deposition of the membrane surface when the operating flux is above the critical flux, it was found that not all the particles convected toward the membrane would eventually deposit onto the membrane surface when the operating flux was above the critical flux. The fractional deposition also varied under different operating conditions of the crossflow microfiltration. (C) 2010 Elsevier B.V. All rights reserved.