The surface degradation of a microlithographically fabricated array of 10 mu m diameter iridium ultramicroelectrodes (UME) was investigated using atomic force microscopy (AFM), Auger scanning electron microscopy (SEM), and stripping voltammetry. Micrometer-size accumulations could be observed on the iridium surface after mercury was electrochemically deposited and stripped approximately five times in a perchloric acid media with a Ag \ AgCl \ 3M NaCl reference electrode. AFM images of the accumulations revealed that they were pyramidal in shape and had a tendency to form in clusters. Elemental analysis of the dusters with Auger SEM identified them as mercury and a small amount of chloride. The accumulations were determined to be mercurous chloride (Hg2Cl2) with the chloride contamination resulting both from leakage of the reference electrode and the decay of the perchloric acid. During subsequent depositions of mercury, chloride ions were eliminated by using a nitric acid media and a polyurethane solid state reference electrode. With chloride eliminated, mercury could be electrodeposited and stripped at least ten times on the iridium UME array without any visible surface degradation.