Multimodal zeta potential distribution in chemical birch pulp process water was studied by filtrating the water into fractions and subsequently measuring zeta potential, charge quantity, turbidity and organic substances. Filtrations were made using 12 μm, 1.6 μm, 1.2 μm, 0.45 μm and 0.1 μm membranes. The number of populations with different zeta potentials diminished with filtration. With the unfiltrated water, 12 μm and 1.6 μm filtrates, three or four different zeta potentials were observed. When the filtration was performed with a 1.2 μm membrane or smaller, only two populations of different zeta potentials were detected. The charge quantity steadily approached zero from unfiltrated water towards a 1.6 μm fraction filtrate. After that, it remained constant. Turbidity constantly decreased when using smaller membranes. The amount of wood extractives decreased to half with 0.1 μm filtration. No significant difference in the amount of carbohydrates and lignin between the filtrated fractions and the unfiltrated water was detected. A comparison dealing with particle size analysis between two different apparatuses was also made.