The thermodynamics for the situation where there is a discontinuous change of the electrode condition with potential due to the formation of a two dimensional condensed liquid-like phase are examined, with particular reference to the effect of the presence of a diffuse layer. The existence of a diffuse layer does not fundamentally change the nature of the transition. However the potential at which the discontinuity is found is very sensitive to the concentration of support electrolyte, although the charge on the electrode is relatively insensitive. It is shown that transitions from gas-like adsorption to liquid-like adsorption are always irreversible (in a theoretical sense) because of the positive edge free energy associated with patches of the liquid-like state.