Deviations from the classic Gouy-Chapman (GC) model due to the finite size of hydrated counterions were tested for negatively charged Langmuir monolayers with different surface charge densities. Monolayers with the largest charge density (> 0.6 C . m(-2)) show an increase of the surface potential for a series of alkali metal cations from Li+ to Cs+ by 200-250 mV. The increase is similar for different monolayers and suggests that this effect is independent of the particular type of headgroup. The magnitude of variation is comparable with model estimations of the electrical double layer (EDL) potential implying that the deviation from the GC model is drastic. Deviations from the GC model rapidly vanish with decreasing monolayer charge density and become hardly observable below 0.3 C . m(-2). For monolayers with a high charge density on subphases containing different sized counterions, preferential participation of the smallest ions in the EDL should be favorable in terms of electrostatic free energy because of packing density limitations. This effect was demonstrated for behenyl sulfate (BS) monolayers (0.64 C . m(-2)) with the X-ray reflectivity technique. For the Cs+- Li+ system, the fraction of Cs+ in the EDL is 50- 60% compared with only 10% of Cs+ in the subphase. Providing high surface charge density, a small univalent Cs+ is capable to compete even with a bulky divalent Mg2+. For equal concentrations of Cs+ and Mg2+ in the subphase, the Cs+/Mg2+ ratio in EDL of BS monolayer is 1.3 to 2.0 ( in contrast to 0.04, predicted by the GC model). All experimental results of this study are described in terms of packing density limitations for hydrated counterions in the EDL.