Aqueous solutions of paraquat used as herbicides mustcorrespond to the FAO Specification Code 56/13/S/6 (FAO, 1973). This requires a description of the active ingredient in the formulation, of the impurities, of the physical and chemical properties, and of the methods for determining the components. The only impurity permitted in paraquat is free 4,4'-bipyridyl at a maximum level of 0.25% of the paraquat content.