A lattice-gas model is considered for ionic adsorption at single-crystal electrodes, the statistical mechanics of which is solved by means of Monte Carlo simulations. The model assumes a short-range nearest-neighbor repulsion and a long-range Coulomb repulsion between adsorbed adions. By fitting the model isotherm to the experimental isotherm for bromide adsorption on Ag(100), which is lattice-gas-like, some conclusions can be drawn regarding the relative importance of these two interaction channels. Various ordered adlayers appearing through second-order order-disorder transitions are found on (100) and (111) surfaces. The inclusion of a simple model for mutual depolarization of the adions leads to first-order phase transitions in the isotherms. Finally some simulations on a uniaxially corrugated (110) surface are considered and compared to experimental results of halide adsorption on Ag(110).