A theoretical basis was given for the linear additive representation of the logarithm of ion-pair solvent extraction constant (K-ex) in terms of logarithmic values of three individual extraction constants (K-cation, K-anion, and K-solvent) for cations, anions, and organic solvents constituting ion-pair extraction systems. The differences in extractability between cations and between anions were quantitatively discussed according to the thermodynamic cycle model of 1:1 ion-pair extraction. The increment of log K-cation and log K-anion of hydrophobic ions is explained by taking the increment of the free energy change of the hydration (Delta G(hyd)) Of the ions into account. The order of the extractability of inorganic cations and anions was also interpreted using literature data of Delta G(hyd) of each inorganic ion. The free energy change of the solvation of a hypothetical ion-pair having a shape of spherical was calculated on the basis of the solvophobic theory. The difference in extracting powers of some extracting organic solvents could also quantitatively be interpreted. There are a few sets of proposed values of the individual extraction constants representing the extractability of the cations and anions, which were calculated based on different reference assumptions. The conclusions of this study an effective for the interpretation concerning the increment of the extractability of the ions irrespective of the reference assumptions for determining the individual extraction constants. It was also attempted to estimate absolute values of K-ex by the theoretical approach in this study. The K-ex values of a few inorganic ion-pairs thus calculated were of the same order of magnitude as those estimated from the individual extraction constants experimentally determined. It is concluded that the thermodynamic cycle model based on the solvophobic theory provides an essential framework for the theoretical interpretation of the mechanism of the ion-pair extraction, and that the validity of the linear additive representation of log K-ex with the three individual extraction constants is demonstrated.