The validity of the simple mean field approximation when it is applied to monolayers adsorbed on electrode surfaces is tested by the Monte-Carlo technique. Using two-, three- and eight-state solvent models it was found that the validity of this approximation depends mainly upon the short-range interaction parameter. If this parameter is close to zero, the mean field approximation gives results that are in excellent agreement with the Monte-Carlo method otherwise the predictions of the mean field approximation exhibit reasonable deviations from those of the Monte-Carlo method over a wide range of molecular parameters. Significant deviations among the results obtained from the mean-field approximation, the quasi-chemical approximation and the Monte-Carlo method may be observed under extreme conditions of verb strong electric fields. Under these conditions there are evidences for the existence of a peculiar phase transition appearing below a critical adsorbate concentration and characterized by two or three transition potentials. Another peculiar phase transition exhibiting just one transition potential is also detected and discussed.