Diffusion coefficients of six common pesticides—cyromazine, chlorotoluron, pirimicarb, metazachlor, tebuconazole and sulcotrione—in water were measured as a function of temperature from 5 to 50 °C using the Taylor dispersion technique. At room temperature (25 °C), the lower diffusivity, 0.35 × 10−9 m2 s−1, is obtained for tebuconazole. For the other studied pesticides, diffusivities are higher, varying at 25 °C from 0.59 × 10−9 m2 s−1 for pirimicarb to 0.73 × 10−9 m2 s−1 for cyromazine. A group contribution method was developed to estimate diffusion coefficients of a larger number of pesticides, leading to a precision of 15%. Diffusion coefficients were then incorporated in a prediction scheme of the fate of persistent pollutants in the environment (fugacity soil model). The precision obtained with the group contribution model was proved to be sufficient for use in this environmental model. The introduction in such a model of an experimental or estimated value for the diffusion coefficient thus different for each pesticide is an improvement compared to the use of a constant value as often proposed in the literature.