Atomic-level microscopies have proved useful to map solid-surface sites directly, but, because of their lack of chemical specicity, they are less adept at identifying unique chemical activity on those sites. Here we present a dual-titration approach developed to probe minority sites on solid surfaces with unique chemical properties of potential relevance to heterogeneous catalysis. Our methodology involves the initial dosing of a chemical probe such as carbon monoxide or ammonia to drive its selective adsorption onto specific sites with particular chemical activity, and the subsequent adsorption of xenon to help identify the nature of those sites. A combination of photoelectron and temperature-programmed desorption spectroscopies are used to characterize the Xe adsorption. The chemistry of oxygen-modifed Ni(110) single crystals was probed to test this technique. It was observed that whereas CO does not discriminate among the various sites present on those surfaces, ammonia binds preferentially to the end of -Ni-O rows and modifies their local electrostatic potential. In addition, it was determined that adsorbed CO aids in a reversible surface reconstruction involving the coalescence of fragmented surface -Ni-O rows at high (> 350 K) temperatures.