SummaryIn glasshouse experiments, additions of 10–100 g 1−1ammonium sulphate enhanced the phytotoxicity to broadleaved weeds and cereals of several water‐soluble herbicides applied post‐emergence in 75–300 1 ha−1with hydraulic nozzles. Studies with dichlorprop potassium salt and chickweedStellaria media(L.) Vill. examined interactions between ammonium sulphate and environmental, application and formulation factors. Simulated rainfall immediately after spraying greatly reduced dichlorprop activity, whether or not ammonium sulphate was present. However, when there was an interval of 2–24 h between spraying and rainfall, the additive increased phytotoxicity. Surfactants tended to reduce dichlorprop phytotoxicity toStellaria media, both in the presence and absence of ammonium sulphate. Certain other inorganic salts including sodium sulphate also enhanced phytotoxicity. Applications by rotary atomizer in very low spray volume (15 1 ha−1, 250–280 μm drops) were less effective than conventional 150 1 ha−1applications. When very low volume application was used, addition of ammonium sulphate or nitrate tended to reduce activity further. In the field, ammonium sulphate significantly increased the effects against weeds of a commercial dichlorprop potassium salt formulation applied conventionally in 200 1 ha−1spray volume. Neutralized phosphoric acid had a similar effect but a mixture of this additive and ammonium sulphate reduced phytotoxicity. Both additives slightly increased dichlo