The photodegradation of atrazine in topsoil under solar light and the factors affecting this have been studied. A rate constant of 0.08-0.17 day(-1), photolytic depth of 0.5 mm and half-life of 4-8 days were determined. The results show that soil granularity, pH value, humidity, organic content, humic acid and surface-active agents could affect the photodegradation of atrazine in soil. (C) 2004 Elsevier B.V. All rights reserved.