Field and growth chamber experiments were conducted to investigate the influence of emulsifiable mineral oil, emulsifiable rapeseed oil, and the emulsifier (polyoxyethylene C13/C15 oxo alcohol) on the efficacy of phenmedipham and metoxuron against common lambsquarters, and of sethoxydim and the ethyl ester of quizalofop against winter wheat. In field studies, mineral oil and rapeseed oil were equally effective in increasing the efficacy of phenmedipham (0.24 and 0.48 kg ai/ha), metoxuron (0.6 and 1.2 kg ai/ha), and sethoxydim (0.05 and 0.1 kg ai/ha), but mineral oil was superior to rapeseed oil with quizalofop. In growth chamber studies, mineral oil and rapeseed oil were equally effective in increasing the efficacy of metoxuron, sethoxydim, and quizalofop-ethyl, but rapeseed oil was more effective than mineral oil with phenmedipham. Both in the field and in the growth chamber, the emulsifier was at least as effective as the oil adjuvants, when combined with all herbicides except sethoxydim. The herbicide doses giving 50% response (ED50) were determined for the growth chamber experiments. The adjuvants reduced the ED50 2.7-fold with phenmedipham, 51-fold with metoxuron, 2.7-fold with sethoxydim, and 6.8-fold with quizalofop when averaged over the three adjuvants. It was concluded that rapeseed oil (all tested herbicides except quizalofop) or the emulsifier (all tested herbicides except sethoxydim) can serve as a substitute for mineral oil.