At present, microfiltration is used to purify the Chinese herb decoction as a new advanced technology, but the fouling leads to the dramatic decline of the permeate flux of membrane. Adsorption is generally considered to be of great effect on the fouling. In order to understand the adsorption phenomena on the surface of ceramic membranes, the adsorption behavior of macromolecules in Chinese herb aqueous extract on ZrO2 particles was studied in this work. The equilibrium isotherm of adsorption obtained at pH 5.0 had good correspondence to the Langmuir model. The adsorption kinetics experiments showed that the rate and mass of adsorption were the largest at pH value of ZrO2 particle's isoelectric point (IEP). A ZrO2 ceramic membrane with the mean pore size of 150 nm was used in the microfiltration of the aqueous extract of a kind of Chinese herb (Radix Rehmannia) at various pH values. The fouling at pH value of ZrO2 particle's IEP was most serious, which resulted in the minimum steady flux and the highest rejection of total solids. This was in agreement with the adsorption behavior of the macromolecules in the aqueous extract onto the ZrO2 particles. (C) 2005 Elsevier B.V. All rights reserved.