Possibility of the determination of activity product (AP) of the compounds of the transuranium element, unavailable in weighable
quantities or with high specific activity has been considered based on studying the cocrystallization of their ultramicro
quantities with isomorphous microcomponents. Conditions of applicability of cocrystallization technique for this purpose have
been discussed. Good coincidence of the values for APNaAmEDTA, calculated from the data on cocrystallization of ultramicro quantities of (AmEDTA)− with isomorphous microcomponent, NaEuEDTA, and APNaAmEDTA obtained directly from the data on dissolution of weighable quantities of NaAmEDTA has been shown.