The decay process of small islands atop Au single crystals was observed by in situ atomic force microscopy in 50 mM H2SO4 aqueous solution at 1050 mV (vs normal hydrogen electrode, where no reaction current was detected). It was found that the area of islands on the terrace decreases linearly with time and the decay rate of the islands in solution is more than 30 times faster than that reported in air. We also found that the island in the same sulfuric solution at 1050 mV decayed more rapidly on Au(100) than on Au(111). We discuss the role of the excess charge at the metal/ electrolyte interface, namely the electric double layer, which may affect the surface diffusion process of atoms atop the electrode in aqueous solution.