A study of inorganic phosphorus in the tidal Hudson River was performed from November 1988 to October 1989. Results indicate that phosphate concentrations are at or near equilibrium with a suspended solid phase consisting of amorphous ferric phosphate in amorphous ferric hydroxide. Equilibrium was observed over most of the river, over most of the year. Undersaturation was observed regularly below river mile 40 and in the month of July. It represented the major deviation from equilibration. Low suspended sediment levels and dilution from sea water are believed to be responsible for undersaturation. Dissolved iron was near equilibrium with amorphous ferric hydroxide except in July. Recognition of the wide range of watersheds where phosphorus equilibrium controls phosphate concentrations suggests that the global, terrestrial flux of biologically available phosphorus may be double current estimates.