The electrosorption of two-base mixtures of adenine, uracil and 1,3-dimethyluracil at the mercury electrode|solution interface has been studied at pH 8.0. Mixtures of adenine or uracil with 1,3-dimethyluracil, i.e., non-complementary bases, exhibit an initial dilute region of adsorption where the bases adsorb in a flat orientation on the electrode surface. At critically defined bulk solution concentrations of adenine or uracil a surface reorientation occurs and the latter compounds adopt a perpendicular surface stance. The presence of 1,3-dimethyluracil has very little effect on the ability of adenine or uracil to undergo the latter surface reorientation process. However, mixtures of adenine and uracil, a complementary base pair, exhibit only a flat surface orientation. Even at bulk solution concentrations considerably greater than those required to observe the flat-to-perpendicular reorientation for the single bases the mixed base system does not exhibit such a surface phenomenon. It is proposed that one of the contributing factors to stabilization of the dilute (flat) adsorption layer in the mixed complementary base system is Watson-Crick hydrogen bonding between the base pairs on the electrode surface.