Abstract: A method for the determination of phosphorus in high purity silicon was described. 32P was separated by ammonium phosphomolybdate from the irradiated silicon matrix and was transformed into MgNH4PO4 6H2O, the activity of which was measured with a Au-Si surface barrier detector. The second-order interference was taken into consideration and the limit of detection in existence of the interference was evaluated to be 7×10-4 ppm.