The size distributions and chemical composition of particles in natural surface waters were studied using filtration and laser light scattering. An on-line tangential sequential-stage filtration system was tested using compounds of known particle sizes. With this filtration system six fractions of different particle sizes were obtained which could then be further analysed, e.g. by ICP-AES, resulting in the chemical composition of each particle size fraction. This technique was then applied to natural surface waters. Data were compared with parallel filtration results using three filters. In addition, the particle size distribution of several European river waters was measured by laser light scattering.