The TOXSWA model has been extended with the functionality to simulate pesticide volatilization from open water surfaces on the basis of a micro-meteorological framework. TOXSWA simulates the fate of pesticides in water bodies to calculate exposure concentrations for aquatic organisms or sediment-dwelling organisms as part of the aquatic risk assessment in the regulatory process of pesticides at EU level and in The Netherlands. Volatilization of pesticides from water to air has been described in the TOXSWA model based on the Liss and Slater method. This report describes the implementation in TOXSWA of an improved methodology to describe volatilization of pesticides from small water bodies. Hourly meteorological data on wind speed and air temperature are used to calculate the rate of volatilization of the pesticide from the water layer.