A method is described for the determination of inorganic phosphate in sea water by extraction of the phosphomolybdate complex into iso-butanol and by reduction in the organic phase with stannous chloride. The color does not fade in 75 minutes. Arsenic and silicate, in amounts ordinarily encountered in sea water, do not interfere. The method is an order of magnitude more sensitive than others· in current use and has the same relative accuracy.