An in situ scanning tunneling microscopy study of morphological changes during formation and reduction of an oxide monolayer on Au(111) in 0.1M HClO4 is presented. During oxidation of a freshly prepared surface, the (1 x 1) arrangement of metal atoms persisted through the hydroxide peak at 1.35 V vs. reversible hydrogen electrode (RHE). At the peak potential for AuO formation (1.55 V vs. RHE), a slightly roughened layer propagated across terraces starting from edges. An ordered oxidized surface structure was not resolved and the surface roughness is approximately 0.08 +/- 0.02 nm. This is ascribed tentatively to the formation of a monolayer of oxide by place exchange. During a slow cathodic sweep, the oxidized surface restructured into wormlike islands of monatomic height at the peak potential for oxide reduction at 1.15 V vs. RHE. These islands coalesced to form flat terraces with monatomic pits in the surface at potentials within the double-layer region. Pits eventually fused with terrace ledges to restore the original terrace morphology.