Methods are described for determining diquat and paraquat, singly and in admixture, in formulations. For determining diquat, ultraviolet absorptiometry a t 310 m p in a sodium acetate buffer solution a t pH 4.05 is adopted.Paraquat is determined in a diluted solution by measuring the optical density, a t 600 mp, of the blue free radical produced by reduction with alkaline sodium dithionite. For analysing mixtures containing both diquat andparaquat, these methods are combined and use is made of a base-line correction procedure to compensate for interference of diquat in the determination of paraquat. The error for these methods is established to be within 3 2 per cent.