Abstract Both malathion and malaoxon are completely extractable from aqueous suspensions by benzene- isobutanol (1 + 1). Aliquots of the extract are evaporated at 110– 120°C, the residue is dissolved in methanol, and the total ferric hydroxamate color is determined. Extraction of a second aliquot with cyclohexane removes malathion and some malaoxon. Malaoxon is selectively partitioned from cyclohexane into alkaline hydroxylamine, leaving only malathion in the cyclohexane phase. After the cyclohexane phase is evaporated, the amount of malathion can be estimated by the ferric hydroxamate method. The difference in the color for the benzene-isobutanol extract and the cyclohexane extract represents that due to malaoxon; the amount of malaoxon may be calculated from a standard curve. The method, sensitive to about 0.2 μmole malaoxon, is useful for monitoring the laboratory preparation of malaoxon from malathion.