Fresh water and sea water usually differ considerably in their Sr/Ca and Mg/Ca ratios. This suggests that these ratios in fossil shells could be used to determine paleosalinities. Calculation of the Sr/Ca and Mg/Ca ratios in mixtures of sea water and average fresh water using a mixing equation shows that the ratios first increase rapidly with salinity to about 100% but subsequently remain nearly constant. The source of most of the Ca, Sr, and Mg in mixtures of fresh and sea water is the sea water, thus except at low salinities the trace element to Ca ratio of mixtures differs little from that in sea water. Analysis both of waters across a spectrum of salinities and of aragonitic mollusc shells growing in water of various salinities conforms to the predicted pattern. However, trace element to Ca ratios offer potential for paleosalinity determination in waters of salinity below about 100%.