An outstanding property of the bipyridylium herbicides is the rapidity with which they are inactivated in contact with soils. The initial inactivation is attributable to strong adsorption on soil colloids and, as may be expected, is governed by the nature of the soil. Subsequent degradation may be induced by ultraviolet radiation or, below the soil surface, by the activity of the microflora. The agronomic significance of these mechanisms is described by reference to recent work on paraquat.