Several organic substances exhibit 2D first-order phase transitions in their adsorption at the metal/electrolyte interface. This phenomenon is discussed on the basis of the Ising lattice gas model. Firstly, its predictions are summed up concerning the critical behaviour of adsorption layers (phase diagrams, critical exponents). Secondly, the mean-field approximation and the quasi-chemical approximation for condensed adsorption layers are investigated critically. In conclusion, low-temperature series expansions of the degree of coverage are found to be adequate isotherms.