Adsorption of polycation as well as the modification with oppositely charged polyelectrolytes is a useful tool for surface modification. As shown previously, a strong enhancement of the attainable specific cationic surface charge was observed, e.g., with poly(diallyl-dimethylammonium chloride) (PDADMAC) as the polycation and poly(maleic acid-co-โฃ-methylstyrene) (P(MS-โฃ-MeSty) as the polyanion, at a ratio of anionic to cationic charges of nฯช/nฯฉ ฯญ 0.6 . . . 0.7. However, because the surface charge seemed to be strongly affected by the conditions of modification, the influence of particle concentration was investigated in detail. One gram of clay was suspended in different volumes of water so that the particle concentration was varied from 1 to 100 g/L. These suspensions were treated under the same conditions. For characterizing the surface charge of clay polyelectrolyte titration, dye adsorption, or microelectrophoresis was used. The comparison between the results of polyelectrolyte titration and the carbon content in the residual solution allowed prediction of the existence of a polyelectrolyte complex and calculation of the stoichiometry. It was found for a constant concentration of PDADMAC per 1 g of clay that the surface charge of modified clays changed from negative (1 g/L) to strong positive, depending on the particle concentration. The cause was assumed to be the differences in the probability of interaction of particles, combined with the changes in the stoichiometry of the polyelectrolyte complex built in the solution, whereas the effect of other factors (conductivity, time of adsorption) was rather low. A strong cationic surface modification was obtained for medium and high particle concentrations only.