Theoretical expressions for thermodynamic parameters of the chemical reaction processes of ionic hydration in seawater are obtained by using the kinetic model of hydrated water molecules around ions in liquid water. Assuming the additivity law between the instantaneous (∞) thermodynamic quantities of electrolytes in seawater and that of the remaining nonhydrated water (i.e., free water), we split the relaxational (r) quantity of seawater from the equilibrium one (0) by substracting the ∞ value. The quantity (∂T/∂P)S,0, which may be related to potential temperature, can be divided into instantaneous and relaxation, respectively. The relaxational quantity (∂T/∂P)S,r always becomes negative, while the instantaneous one (∂T/∂P)S,∞ is always positive. Both absolute values of (∂T/∂P)S for r and ∞ decrease with increasing salinity and temperature, respectively. This fact is illustrated in terms of the ionic hydration.