The degradation rate of plant protection products and their transformation products in surface water and sediment may influence their concentrations in Dutch surface water. Therefore the estimation of these rates may be an important part of the assessment of the exposure of aquatic organisms. We propose a stepped sequence of studies for estimating the rate in water going from simple and conservative to more sophisticated and more realistic studies. The sequence includes: - studies on hydrolysis and photolysis; - studies with fresh surface water in the dark; - water-sediment studies in the dark or in light; - studies with algae and macrophytes; - outdoor studies in realistic surface water systems. The usefulness of these studies for the exposure assessment in Dutch surface water is discussed.