A spectrophotometric method for determination of Paraquat, based on its reaction with BiI(-)(4) ion in the presence of gum arabic, is described. The apparent molar absorptivity of the product is 1.64 x 10(4) 1.mole(-1).cm(-1) at 515 nm. Results for the analysis of commercial herbicides are in good agreement with those obtained by the dithionite method.