UV-cleaned gold substrates incubated in solutions of alkanethiol show islands on the monolayer surface when imaged with noncontact atomic force microscopy (AFM). The height of the islands above the monolayer is approximately twice the height of the alkanethiol monolayer, and the diameter of the islands is 20-200 nm. These islands are easily pushed aside during contact mode AFM imaging without damaging the underlying monolayer. Islands are observed on gold substrates exposed to solutions of octadecane-, hexadecane-, and dodecanethiol and 1＇-thiahexa(ethylene oxide)-1-octadecane at 0.01-1.0 mM concentrations in ethanol and hexadecane. AFM on samples with submonolayer coverage shows that the islands are not observed until the late stages of monolayer formation. Islands are not observed on freshly deposited gold substrates or on UV-cleaned gold substrates that are exposed to ethanol for longer than 10 min prior to incubation in alkanethiol solutions. We conclude that the island formation is associated with oxidation of the gold surface and that the islands are primarily composed of alkanethiol. We hypothesize that the stability of these structures may be due to the formation of multimolecular complexes of alkanethiols.