Chemical processes often produce filtrate fractions with different chemical and physical properties. Typically filtrate fractions differ in the concentration of dissolved reaction products, reagents, and salts, but they may also differ in pH and temperature. In fractional filtrate configurations these fractions are used separately in earlier stages of the process in a way that ensures the best possible responses for the process.The purpose of this paper is to make a theoretical and systematic evaluation of the possibilities of different fractional filtrate configurations, i.e. fractional washing concepts, in connection with the improvement of wood pulp fibre bleaching. A washing model was used to simulate dissolved organic and residual chemical material balances in eight different process configurations. Each process configuration represents one single bleaching stage, consisting of a filter washer, a reactor, and a second filter washer in series.The results demonstrate that fractional filtrate configurations have the potential either to increase washing efficiency for dissolved reaction products, or to increase the recycling ability of the active process chemicals. However, these two goals are contradictory. The results encourage the creation of process machinery and configurations where different filtrate fractions are used differently for specific aims. The idea of fractional filtrate configuration should also be applicable to and beneficial for other branches of the chemical and mineral processing industries. (C) 2003 Elsevier Science B.V. All rights reserved.