X-ray diffraction techniques have been used to determine the structures of Tl-CI, Tl-I, and Pb-Br compounds adsorbed on Au(111) in acid solution. As in the previous case of Tl-Br, several well-defined mixed adlayer phases with stoichiometries of 1:1, 1:2, and 1:3 have been found over a wide potential region. Sufficient data have been gathered to explore the general structural trends among electrochemically prepared two-dimensional (2D) binary ionic compounds. The observed structural phase behavior is rationalized by symmetry and electrostatic considerations in 2D binary ionic crystals. For the 1:1 stoichiometry, square symmetry is generally preferred because it allows an ion to surround itself with the maximum number of counterions. On a hexagonal substrate, this becomes distorted to either a rectangle or a rhombus in commensurate or uniaxially commensurate phases, depending on whether the cation and anion layers are in the same plane.