In order to well elucidate the true nature of filter cake formed in membrane filtration of aqueous colloids, the transient flux decline behaviors accompanied with step-up and step-down changes in the applied filtration pressure during the course of constant pressure filtration were investigated by carrying out both microfiltration experiments of kaolin slurry and ultrafiltration experiments of nanosilica sol under various experimental conditions. When the applied pressure was increased stepwise, for each colloid the plot of the reciprocal filtration rate vs. the filtrate volume per unit membrane area referred to as the Ruth plot tends to approach a linear relationship obtained in constant pressure filtration conducted under the increased pressure condition from the beginning of filtration, after a transient period when the preformed cake under the initial pressure was compressed. In contrast, in the case of the step-down change in the applied pressure, when the volume of the preformed cake is adequate, the plot after a transient period tends to approach a higher straight line parallel to a linear relationship for constant pressure filtration conducted under the decreased pressure condition from the beginning. Such flux decay behaviors were well described by the expression having two constants: one is the constant representing the final filterability for the preformed filter cake, and the other is the rate constant describing the change rate in the structure of preformed cake resulting from a pressure jump. Moreover, the influence of experimental conditions on the cake reversibility and the rate constant was revealed both for kaolin slurry and for nanosilica sol. (C) 2016 Elsevier B.V. All rights reserved.