The extraction equilibrium of pyrocatechol from an aqueous solution by the trioctylamine sulfate salts (TOA salts) in an organic solvent was investigated. TOA salts were first formed, and then pyrocatechol was extracted into the organic phase from the aqueous phase by the TOA salts. The modified model of Wilson, which accounts for equilibrium of the chemical reaction of trioctylamine and sulfuric acid to form various TOA salts, was applicable to any reaction systems to predict the extraction of pyrocatechol. The number of water molecules near the various TOA salts, which reflects the extraction capability of pyrocatechol, was determined. The results obtained from the modified model of equilibrium of the chemical reaction of trioctylamine and sulfuric acid are fairly consistent with the experimental data. The effects of the initial concentration of TOA, inorganic acid, and organic solvents on the distribution coefficient of pyrocatechol were investigated in detail.