Theoretical charge density-potential isotherms have been derived for anodic monolayers of honeycomb, square, and triangular lattice geometry on the basis of the Ising lattice model. For mercury sulfide monolayers at the mercury/electrolyte interface, the charge-potential isotherms have been measured in the temperature region between 279 and 313 K. The critical temperature of the monolayer was indicated by a sudden change in the isotherm shape. From the critical temperature, the lateral interaction energy of the molecules in the HgS monolayer has been directly calculated. On the basis of the Ising lattice gas model, these isotherms can be theoretically well described with a low-temperature series expansion of the degree of coverage with respect to the activity of the solution phase species. The adsorption energy and number of electrons involved in the electrode reaction obtained are practically independent of temperature.