The Pitzer equations have been used to fit the apparent molal adiabatic compressibility of electrolytes in water as a function of temperature. The equations have been used to estimate the partial molal compressibility for a number of cations and anions in 0.725 m NaCl and average seawater (S = 35 g kg(-1)). The calculated results for electrolytes are in good agreement with direct measurements in 0.725 m NaCl and seawater. The partial molal compressibilities estimated from the model can be used to make more reliable estimates of the effect of pressure on activity coefficients and ionic equilibria in seawater and other mixed electrolyte solutions at high pressures.