Abstract A procedure is given for the determination of radiophosphorus in the presence of other activities. Potassium phosphate is added as carrier and the phosphorus is extracted as phospho-molybdic acid into a butanol-chloroform mixture. The activity is back-extracted into ammonium hydroxide and the phosphate is precipitated as magnesium ammonium phosphate. This may be air-dried or ignited to magnesium pyrophosphate for counting. The method is rapid, accurate, and free from interference by other radionuclides. Especially noteworthy is the excellent separation from radioarsenic.