The stability constants K 1 and K 12 of phosphoric acid were determined for artificial seawater of six different salinities (5, 10, 20, 30, 35, 40‰) and in 0.2, 0.4 and 0.7 M NaCl at three temperatures (5, 15, 25°C). The results are compared with those of Kester and Pytkowicz (1967), Atlas et al. (1976) and Dickson and Riley (1979). The ionic product of water, K w , was determined in sodium chloride media at 5, 15 and 25°C. Complex formation among Ca 2+ , Mg 2+ , HPO 4 2− and PO 4 3− is discussed.