The structure of CdS monolayers electrodeposited onto Au(111) substrates was studied using scanning tunneling microscopy (STM) and electrochemistry. The films were built by sequential deposition of a partial monolayer of S atoms and Cd atoms onto Au(111). We report micrometer-scale and atomically resolved STM images of the S/Au(111) and the CdS/Au(111) surfaces. The S/Au(111) surface is atomically flat over several square micrometers and is composed of uniform, single-crystal terraces. The adlayer unit cell was found to be (square-root 3 x square-root 3)R30-degrees with respect to the underlying Au(111), with a S-S distance of 0.50 nm. The CdS/Au(111) surface was also found to be composed of well-ordered, atomically flat terraces on the micrometer scale. Atomic resolution images of the CdS surface revealed a single 6-fold symmetric structure with an interatomic spacing of 0.43 nm. A model for the structure of the CdS monolayer is proposed and discussed. The Au(111) substrate exerts a pronounced template effect which is attributed to the strong interaction between S and Au in this system.