A method for preparation and analysis of the oxygen isotope composition (δ¹⁸O) of dissolved inorganic phosphate (DIP) has been developed and preliminary results for water samples from various locations are reported. Phosphate is extracted from seawater samples by coprecipitation with magnesium hydroxide. Phosphate is further purified through a series of precipitations and resin separation and is ultimately converted to silver phosphate. Silver phosphate samples are pyrolitically decomposed to carbon monoxide and analyzed for δ¹⁸O. Silver phosphate samples weighing 0.7 mg (3.5 µmol oxygen) can be analyzed routinely with an average standard deviation of about 0.3‰. There is no isotope fractionation during extraction and blanks are negligible within analytical error. Reproducibility was determined for both laboratory standards and natural samples by multiple analyses. A comparison between filtered and unfiltered natural seawater samples was also conducted and no appreciable difference was observed for the samples tested. The δ¹⁸O values of DIP in seawater determined using this method range from 18.6‰ to 22.3‰, suggesting small but detectable natural variability in seawater. For the San Francisco Bay estuary DIP δ¹⁸O is more variable, ranging from 11.4‰ near the San Joaquin River to 20.1‰ near the Golden Gate Bridge, and was well correlated with salinity, phosphate concentration, and δ¹⁸O of water.