The two-dimensional condensation behavior exhibited by nucleobases at a mercury/aqueous solution interface is analyzed on the basis of their hydrogen-bonded quadruplex structures, and the experimentally observed critical temperatures are rationalized incorporating different types of hydrogen bonding, surface coordination effects, and stacking considerations. The proposed methodology provides a structural basis for the interpretation of critical temperatures and enables the calculation of the same pertaining to different modified nucleobases. The applicability of the procedure to order-disorder transitions of water dipoles at Hg electrodes is also indicated.