Terbuthylazine, a non-selective herbicide developed by Ciba Geigy, is a triazine herbicide, as are simazine, terbutryn and atrazine. Terbuthylazine is a broad-spectrt~n herbicide registered for control of most annual grasses and broadleaf weeds in rubber, oil palm and cocoa plantations and in orchards as well as on non-crop land. This herbicide was introduced ir~to the Malaysian market in the 1980's under the name of Gardporin~'. The persistence of triazine in soil depends on the rate of application and such envir~tal factors as soil moisture, temperature, pH and constituents (WSSA 1989). Weber and Weed (1974) reported that higher soil temperature and moisture enhance degradation of triazine, and that adsorption and bioactivity of the herbicide was correlated with organic matter and clay content and other soil parameters. Organic matter was the primary adsorbing surface in the soil (Weber and Weed 1974; Nishimoto and RaW, ran 1985). The influence of organic content of the soil is illustrated by the fact that the half-life of simazine ranged from 22 to 44 days, depending on organic content of the soil (Walker and Th~son 1977). Degradation of atrazine has been reported to be faster in moist soil than in dry soil (Walker and Zimdahl 1981). The active ingredients of triazine are stable chemical compounds which, however, degrade rapidly in soil due to microbial activity (WSSA 1989). The maximum movement of terbuthylazine under natural rainfall was 20 am after 21 days (Bowman 1989). While much is known about the activity and mode of action of other compounds in the triazine group, little quantitative information is available concerning the mode of dissipation of terbuthylazine from soil, especially under tropical conditions. Experiments were therefore conducted to determine the persistence of terbuthylazine at various moisture levels and temperatures in two different soil types.