The complex formation between dodecylpyridinium chloride (DPC) and multicharged anionic planar substances, 14 azo dyes and 3 benzene- or naphthalenesulfonates, has been studied by the potentiometric titration using a surfactant selective electrode. The agreement between the observed maximum binding number and the number of anionic charges (n) on dye molecules showed n:1 complex formation. The binding isotherms were found to be composed of two types of binding; one is the noncooperative binding observed at low surfactant concentrations and the other is the cooperative binding at the higher concentrations. The microscopic binding constant for the noncooperative binding was found to take the values in the range of 50-200 mol(-1) dm(3) for many of the substances, but, takes more large values up to 2500 mol(-1) dm(3) for the substances which have a large hydrophobic part or the structure of separate hydrophobic and hydrophilic regions. A multiple regression analysis showed that the data of the corresponding standard free energy change of binding were well interpreted by the equation (in unit of kJ mol(-1)) AGdegrees = -5.85 log P-S -1.68 log P-D -2.12z + 28.4, where Ps and P-D are the partition coefficients of the surfactants and planar substances in the 1-octanol/water system and z is the number of anionic charges on the planar molecules. At the beginning of the cooperative binding, precipitate formation was observed for almost all of the present systems. Among these, some of the dyes having the structure of separate hydrophobic and hydrophilic regions formed a needlelike crystal, which was accompanied by a hysteresis phenomenon in the binding isotherm. The stable complex formation by both the hydrophobic and electrostatic interactions between the surfactant and the planar substances was found to be important for the crystal formation. Depending on the manner of arrangement of the charged groups on the planar substances, the origin of the binding cooperativity was ascribed to the interactions between surfactants bound to one planar-substance molecule or to the association of the complexes. It was also found that the present small binding systems are useful as the model of ligand binding to protein local structures.