In-situ scanning tunneling microscopy (STM) coupled with cyclic voltammetry was used to examine the adsorption of carbon monoxide (CO) molecules on an ordered Au(111) electrode in 0.1 M HClO4. Molecular resolution STM revealed the formation of several commensurate CO adlattices, but the (9 x root3) structure eventually prevailed with time. The CO adlayer was completely electrooxidized to CO2 at 0.9 V versus RHE in CO-free 0.1 M HClO4, as indicated by a broad and irreversible anodic peak which appeared at this potential in a positive potential sweep from 0.05 to 1.6 V. A maximal coverage of 0.3 was estimated for CO admolecules from the amount of charge involved in this feature. Real-time in-situ STM imaging allowed direct visualization of the adsorption process of CO on Au(111) at 0.1 V, showing the lifting of (root3 x 22) reconstruction of Au(111) and the formation of ordered CO adlattices. The (9 x root3) structure observed in CO-saturated perchloric acid has a coverage of 0.28, which is approximately equal to that determined from coulometry. Switching the potential from 0.1 to -0.1 V restored the reconstructed Au(111) with no change in the (9 x root3)-CO adlattice. However, the reconstructed Au(111) featured a pairwise corrugation pattern with two nearest pairs separated by 74 +/-1Angstrom, corresponding to a 14% increase from the ideal value of 65.6Angstrom known for the (root3 x 22) reconstruction. Molecular resolution STM further revealed that protrusions resulting from CO admolecules in the (9 x root3) structure exhibited distinctly different corrugation heights, suggesting that the CO molecules resided at different sites on Au(111). This ordered structure predominated in the potential range between 0.1 and 0.7 V; however, it was converted into new structures of (7 x root7) and (root43 x 2root13) on the unreconstructed Au(111) when the potential was held at 0.8 V for ca. 60 min. The coverage of CO adlayer decreased accordingly from 0.28 to 0.13 before it was completely removed from the Au(111) surface at more positive potentials.