The present invention provides a method for the purification of 227Th from a mixture comprising 227Th and 223Ra, said method comprising: i) preparing a first solution comprising a mixture of 227Th and 223Ra ions dissolved in an aqueous solution of first mineral acid; ii) loading said first solution onto a strong base anion exchange resin; iii) eluting 223Ra from said strong base anion exchange resin using a second mineral acid in an aqueous solution; iv) optionally rinsing said strong base anion exchange resin using a first aqueous medium; v) eluting 227Th from said strong base anion exchange resin using a third mineral acid in an aqueous solution whereby to generate a second solution comprising 227Th. The invention further provides a purified 227Th solution, a corresponding pharmaceutical formulation and methods of treatment of neoplastic disease.