The three approaches to defining pH scales for use in sea water: the N.B.S. scale, the pH(SWS) or 'total' hydrogen ion concentration scale and the 'free' hydrogen ion concentration scale are described, and it is shown how these arise as a direct consequence of alternative experimental procedures for determining practical acidity constants. The advantages of conceptual simplicity and of experimental precision inherent in the use of concentration products to describe proton-transfer reactions in saline media are emphasised. In addition, the problems of theoretical interpretation and of reproducibility which result from the conventional nature of the N.B.S. pH scale are described, and the effect on the corresponding 'apparent' constants outlined. Insofar as it is concentration products rather than 'apparent' constants that are amenable to prediction using models for activity coefficients, the deliberate use of a 'free' hydrogen ion concentration scale should be applicable to many areas of aqueous geochemistry in addition to marine chemistry.