The relationship between surface activity and weed control efficacy of diuron spray solution (0.075 kg ai/ha) with 12 adjuvants (0.1% v/v) was studied in the laboratory and the greenhouse. Organosilicone adjuvants reduced surface tension (ST) and contact angle (CA) of diuron spray solutions to a greater extent than did nonsilicone adjuvants. Three organosilicone adjuvants significantly increased the control and reduced the fresh weight of barnyardgrass. Contact angle was more closely correlated with diuron efficacy than ST, and the coefficient of determination between the CA and the fresh weight or control percent of barnyardgrass was 0.41. Kinetic® applied before or in combination with diuron significantly enhanced diuron efficacy. However, Kinetic sprayed after diuron did not affect the efficacy. Kinetic influenced the retention and absorption of diuron rather than its deposition and translocation.