The responses of zeta potential of membrane, zeta(obs), to the pore blocking and cake formation during microfiltration of latex particle suspension were studied by monitoring both the filtration resistance and streaming potential during the filtration. The pattern of the change in zeta(obs) during the filtration was quite different between the cake filtration and the pore blocking filtration. In the cake filtration zeta(obs) changed from the intrinsic zeta potential of membrane, zeta(m), to the zeta potential of cake layer, zeta(c), depending on the proportion of the hydraulic resistance of the cake layer to the total filtration resistance. In the pore blocking filtration followed by the cake filtration zeta(obs) jumped from zeta(m) to a certain value between zeta(m) and zeta(c) just after the start of the filtration and became almost stable. The zeta potential at the blocked pore, zeta(b), depended on the gap between surfaces of the blocked pore and the blocking particle and indicated the extent of pore blocking. It was confirmed that the change in zeta(obs) during filtration will reflect both the location of pressure drop and the local zeta potential. (C) 2012 Elsevier B. V. All rights reserved.