The apparent molal volume V-phi and adiabatic compressibility kappa(phi) of H3PO4, NaH2PO4, Na2HPO4, and Na3PO4 have been determined from density and sound speed measurements in 0.725 m NaCl solutions at 25 degrees C. The partial molal volumes (V) over bar*(i) and compressibilities (kappa) over bar*(i) have been determined by extrapolating the values of V-phi and kappa(phi) to infinite dilution in the NaCl media. The results have been compared to earlier measurements made on a number of electrolytes in NaCl media. The phosphate results showed correlations between the values of (V) over bar*(i) and (k) over bar*(i) as found for other electrolytes. The values for the volume Delta V-i and compressibility Delta kappa(i) changes for the dissociation of H3PO4 in 0.725 m NaCl have been calculated from the measurements. The calculated values of Delta V-i and compressibility Delta kappa(i) have been used to estimate the effect of pressure K-p/K-0 on the dissociation of H3PO4 in the oceans using RT ln[K-P/K-0] = Delta ViP + 0.5 Delta kappa P-i(2), where P is the applied pressure in bars. The results are in reasonable agreement with directly measured values.