A method for chemically differentiating the surface of a set of small, closely-spaced, lithographically-defined Au features on a die, from another set of similar features intimately inter-dispersed, is described. The key enabler of the method is a standard electronics probe array adapted to carry out electrochemistry on the features. The probe array is used first to verify the electrical integrity of features and the quality of electrical contacts by measuring electrical resistance, then, in the presence of the electrolyte, simultaneously maintain one potential on one set of Au features and another potential on the other set in order to carry out desired electrochemical reactions. The technique was demonstrated on dies bearing 40 electrically isolated Au features (based on 5 mu m wide stripes) accessed via 64 contact pads each 100x100 mu m(2) in area. The array had 64 probes, of which 16 were maintained at a desorbing potential (-1.6 V vs. Ag/AgCl) and 48 at a stability potential (-0.3 V). The surface compositions were analyzed with time-of-flight secondary ion mass spectrometry by imaging ion fragments characteristic to the thiols forming SAMs, thereby validating the process. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.087203jes] All rights reserved.