We have obtained the first in situ STM molecular image of a CO adlayer on a Pt(110)-(1 x 1) electrode surface in 0.1 M HClO4 solution. The observed CO adlayer formed an ordered (2 x 1)-2CO structure at saturated coverage. The CO molecules were found to adsorb on top of each Pt surface atom; however, they were tilted with a zigzag configuration along the atomic rows because of the dipole-dipole repulsion of neighboring CO molecules. The high activity of the Pt(110) electrode for surface CO oxidation can be attributed to the low packing density of the adsorbed CO molecules as well as their tilted orientation.