Monte-Carlo simulations of the electric double layer are performed for two electrolyte mixtures next to a plane, uniformly charged, surface. Simulations are made at parameters corresponding to a Poisson-Boltzmann theory which is corrected to include the excluded volume effects of the ions. The corrected Poisson-Boltzmann theory is found to have some deficiencies. The structural properties disagree with simulation results while the theory can be adjusted to give agreement with experiment for the relative concentration excesses of small univalent counterions. (C) 2008 Elsevier Inc. All rights reserved.