By using a high-quality Ag(111) film covered with a bromide monolayer (a monolayer model of the AgX surface) as the substrate well-defined solubility equilibria were observed for the first time for the two-dimensional (2D) J-aggregate of pseudoisocyanine (PIC) perchlorate in both aqueous and ethanolic solutions. The 2D J-aggregation isotherms gave a sharp rise at a well-defined critical dye concentration or threshold reflecting the characteristic solubility of the PIC 2D J-aggregate, which was several orders of magnitude smaller than that known for the homogeneous J-aggregation in a far more concentrated aqueous PIC solution. The results allowed quantitative determination of the solubility product and the related thermodynamic functions. The measured enthalpy of dissolution, together with the approximate solvation energies of PIC cations and ClO₄⁻ anions in the polar solvents, suggested an absolute molar cohesive energy of the PIC 2D J-aggregate of around ∼500 kJ/mol. This large cohesive energy supports the capability of PIC cations and counter anions to form a stable 2D ionic crystal. Experimental hints are also provided as to how the solubility of such a 2D ionic crystal is affected by its physical size and by an excess concentration of common ions in solution.