Phosphorus is selectively extracted as molybdovanadophosphoric acid (MVPA) and determined indirectly using52V. Silicon, germanium, arsenic, fluoride and tannic acid do not interfere. The sensitivity for phosphorus was 1421±13 counts52V per ng P ml–1 with a detection limit of 0.2 ng P ml–1. The precision is ±10% over the range 0.7±70 ng P ml–1. The sensitivity is four orders of magnitude greater than for conventional neutron activation analysis (NAA) using the31P(n, )28 Al reaction and a SLOWPOKE II reactor. Greater selectivity for phosphorus is obtained than by direct measurement of32P by beta ray spectroscopy.