In the wet-chemical treatment of polyimide (PI) membranes with aminic modifiers, the modifier molecules will be covalently bound to the membrane polymer. Using modifiers with high aminic nitrogen the amination is combined with a degradation process, which shifts separation properties from ultrafiltration characteristics (untreated membrane) to microfiltration characteristics (treated membrane). Under optimal treatment conditions the steepness of separating curves of the aminated membranes is comparable with the steepness of separating curves of commercial microfilters. Characteristic data of membranes such as water permeability, amine content, SEM morphology, wettability and the dependence of membrane thickness on treatment conditions were presented. Data show that membrane properties are insensitive to modifier concentration in the treatment bath with respect to degradation reaction but sensitive to reaction rate. From the data of amine content per unit surface area of membrane and amine content per unit membrane weight, it was concluded that the functionalization/degradation process is divided in two steps. In the first step functionalization dominates, whereas in the second step functionalization and degradation are simultaneous processes resulting in an equilibrium state. The expected reaction sequence of degradable functionalization is proposed and discussed. Initial membrane morphology seems to be the key parameter for further investigations to optimize membrane preparation processes. (C) 2003 Elsevier B.V. All rights reserved.