The spatial structure of compressed carbon monoxide adlayers on Pt(111) in aqueous acidic solution has been explored by means of in-situ scanning tunneling microscopy (STM) along with infrared reflection-absorption spectroscopy (IRAS). Besides offering a detailed structural picture of this electrochemical interface in comparison with the well-studied Pt(111)/CO system in ultrahigh vacuum (uhv) environments, the real-space structural information provided by STM allows an assessment of the obfuscating influence of dynamic dipole coupling upon IRAS binding-site assignments. In turn, the latter data provide an important crosscheck on the validity of binding-site assignments deduced from the STM images. Emphasis is placed on the structures formed from near-saturated CO solutions, encouraged by the electrode potential-induced adlayer phase transition at ca. O V vs SCE observed previously under these conditions by IRAS. At potentials below O V, a hexagonal close-packed (2X2)-3CO adlayer is observed, with a CO coverage, theta(CO), of 0.75. The z-corrugation pattern evident in the STM images indicates the presence of two threefold hollow and one atop CO per unit cell. This binding-site assignment is supported by the corresponding IRAS data which yield C-O vibrational bands at ca. 2065 and 1775 cm(-1). The relative intensities of these two v(CO) bands, ca. 2:1, differs markedly from the 1:2 binding site occupancy deduced from STM. This apparent disparity, however, can be accounted for by dynamic dipole coupling effects between the atop and multifold CO oscillators. At potentials above O V (up to the onset of CO electrooxidation at ca. 0.25 V), a markedly different adlayer arrangement is formed, having a (root 19X root 19)R23.4 degrees-13CO unit cell, with theta(CO)=13/19. This hexagonal structure features CO binding in predominantly asymmetric sites inbetween atop and bridging geometries. A distinction between several alternate adlayer arrangements sharing (root 19X root 19) symmetry was achieved on the basis of the z-corrugation pattern along with the corresponding IRAS data upon consideration of dipole-coupling effects. Another CO adlayer structure, having a (root 7X root 7)R19.1 degrees-4CO unit cell (theta(CO)=4/7), was commonly observed al potentials below 0.2 V after the removal of solution-phase CO. These adlayer arrangements are distinctly different to the compressed Pt(111)/CO structures found in uhv. The increased accommodation of CO in multifold sites observed for the former can be understood chiefly from the markedly (ca. 1 V) lower surface potentials (and excess electronic surface charges) characterizing the electrochemical interface.