Dissolved natural or modified chitosans are used as the carriers of bioactive compounds which demands knowledge on how these macromolecules bind chemicals in solutions. This work examines binding of ionizable salicylic acid (SA), a medicine chemical and plant hormone, to chitosan and its derivative obtained by amine N alkylation with n-C₆H₁₃. Binding was studied at two pHs, below the chitosan's pK_a, by performing dialysis with a 3500 Da cut-of membrane, followed by a spectrophotometric determination of SA concentrations in the membrane-separated compartments. SA-chitosan binding obeys reversible linear isotherms showing distinct preferences for SA to distribute from water to macromolecules with a coefficient reaching up to few hundreds (L kg⁻¹), depending on pH and NaCl concentration. Both SA molecules and anions bind to dissolved chitosan. Although the SA anion-chitosan interactions tend to dominate, they can be suppressed by competing anions. Chitosan N-alkylation is associated with a modest decreasing effect on SA binding. For water-soluble formulations of bioactive substances, binding quantification may help to differentiate between the effects of concentration of chitosan-bound active chemicals and the ability of complexes to pass biological barriers.