pH measurements of seawater are suffering of a lack of spatiotemporal compatibility and comparability. The present paper makes an analysis of the current state of art of seawater pH measurement. The theoretical problemes, such as the definition of the activity coefficient, and practical problemes related especially to the complex matrix are pointed out. The necessary actions to overcome theses difficulties are also described. They are part of an European Joint Research Project entituled “Metrology for the salinity and ocean acidification” and funded by EMRP. Some preliminary results obtained for a saline solution containing only NaCl are presented in order to show the influence of the matrix on the experimentally obtained acidity functions.