SummaryIn pot experiments, mixtures of ammonium sulphate with surfactants or oil additives increased the phytotoxicity of commercially formulated bentazone (‘Basagran’) toStellaria media(chickweed). Ammonium sulphate with a proprietary oil adjuvant Actipron had similar effects with benazolin potassium salt, but not with an ethyl ester formulation of benazolin. The phytotoxicity of bentazone and benazolin salts toTrifolium repens(white clover) andLolium perenne(perennial ryegrass) was almost unaffected by these additives. In a field experiment, a mixture of ammonium sulphate with Actipron improved the control ofS. mediaby bentazone and benazolin salts.Oils and surfactants markedly increased rates of entry of14C bentazone into leaves ofS. media, white clover andChrysanthemum segetum.Ammonium sulphate sometimes had similar effects but on other occasions reduced uptake. In some circumstances the additives apparently interacted synergistically, to increase uptake of labelled herbicide into the leaf or to enhance its transport within the plant. In these test species, differential absorption of bentazone could not explain differences in species susceptibility, suggesting that the main cause of resistance was the ability of plants to degrade the herbic