A new set of pH-scales based on synthetic sea water is proposed. It is consistent with an activity scale for H + chosen so that the activity coefficient approaches unity when the concentration of H + approaches zero in pure synthetic sea water of a certain salinity. An experimental method for assignment of pH-values according to these scales to a set of standard buffers is described. The standard buffers are composed of 'tris' (2-amino-2-hydroxymethylpropane-1,3-diol), 'tris'-hydrochloride and synthetic sea water. The pH-values of the standard buffers in the salinity range of 10-40% and temperature range of 5-30°C are given in Table 6. The suggested pH-scales are consistent with new sets of acidity constants for carbonic acid and boric acid ( HANSSON, in press, a,b,c).