BACKGROUND: Although co-formulants constitute a substantial portion of the total plant protection product (PPP) mass applied to crops, data on residue formation and the behaviour of these substances on plants are scarce. In an earlier study we demonstrated that co-formulants commonly used in PPPs can form considerable residues, i.e., in the low to medium mg/kg range, but normally decline rapidly within few days. In the field trial reported here, we aimed to identify the major decline processes of co-formulants. Residues of co-formulants were therefore monitored in parsley and lettuce grown in an open field as well as under foil tunnels equipped with either an overhead or a drip irrigation system. RESULTS: Dissipation of three anionic surfactants was markedly faster when crops (parsley and lettuce) were exposed to natural rainfall or irrigation from above compared to drip irrigation. In contrast, the decline of three volatile organic solvents was not affected by rain or irrigation, but was dependent on the crop, with much shorter half-lives in lettuce than in parsley. Furthermore, dilution through plant growth contributed significantly to the reduction of residues over time. CONCLUSION: In this work we substantiate earlier findings on the magnitude and dissipation of residues of anionic surfactants and solvents representing the most important co-formulant classes. The chosen experimental setup allowed differentiation between decline processes and we confirm that foliar wash-off is a major dissipation process for anionic surfactants. For volatile organic solvents, dissipation appears to depend on the properties not only of the substance but also of the plant (surface).