Monte Carlo (MC) simultations have been performed for an ionic system between two parallel uniformly charged plates modelling the conditions found in lamellar lyotropic liquid crystals [1] (Fig. 1). The results are used to assess the accuracy of the statistical mechanical approximations leading to the Poisson–Boltzmann (PB) equation. In general there is a good agreement between the MC simultations and results obtained from the PB equation [2], both in the system studied here and in analogous systems [3]. This applies to the ion distribution (Fig. 2) Fig. 1. The amphiphilic molecules form bilayers which are intercalated with aqueous lamellae also containing the counterions. The upper figure shows the schematic model of the aqueous region used in the MC simulations Fig. 2. Concentrations profiles between the two charged plates for mono- and divalent ions. The plate distance 2b = 21.0 Å. The smooth curves are taken from the PB equation and the two other are obtained from MC-simulations. and to a lesser extent to the energy. In particular the PB equation predicts the correct asymptotic behaviour as the distance between the plates is increased. In the MC simulations there is slight increase in the ion concentration close to the walls compared to the PB solution. This is due to the fact that in the MC simulations one allows for correlations between the ion while the PB equation is derived under assumption that the pair correlation function is constant. It is shown that the accumulation of ions close to the walls is solely due to the repulsions between the ions and it is argued that ion binding in polyelectrolyte system can be understood as caused by the incapability of the bulk solution to sustain a substantial net charged over moderately large distances.