Several herbicides were tested in the greenhouse on ivyleaf morningglory (Ipomoea hederacea (L.) Jacq.), green foxtail (Setaria viridis (L.) Beauv.), purple nutsedge (Cyperus rotundus L.), and quackgrass (Agropyron repens (L.) Beauv.) to determine the degree of enhancement in activity that could be obtained with an isoparaffinic oil carrier applied at 140 L/ha. The enhancement varied with the herbicide and with the species, ranging from 16-fold enhancement with 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) and 2-sec-butyl-4,6-dinitrophenol (dinoseb) on ivyleaf morningglory to no enhancement of atrazine activity on purple nutsedge and quackgrass or (2,4-dichlorophenoxy)acetic acid (2,4-D) activity on quackgrass and ivyleaf morningglory. An oil adjuvant was less effective in enhancing dinoseb and 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (linuron) activity than was the isoparaffinic oil carrier. Also, the isoparaffinic oil carrier emulsified in water was less effective than the undiluted oil in enhancing dinoseb activity on green foxtail, even though equal volumes of the isoparaffinic oil were applied.