When chemisorbed oxygen on silver at full coverage was exposed to carbon monoxide at 273 K, a small amount of the chemisorbed oxygen reacted readily with carbon monoxide, but the rest of the oxygen remained inert. However, when some oxygen gas was added to the ambient carbon monoxide, the reaction to form carbon dioxide proceeded at a remarkable rate. The rate of the reaction in the presence of both oxygen and carbon monoxide in the ambient gas was first order with respect to carbon monoxide, and zero order with respect to oxygen gas. Such behavior suggests that the reaction between oxygen and carbon monoxide takes place in a very limited part of the silver surface, all the rest part remaining unreactive, and both active and unreactive regions being fully covered by oxygen during the reaction. A method to identify the limited number of active sites on the surface is suggested using scanning tunneling microscopy (STM) at nanoscale resolution. It is in conjunction with the isotope jump method at condition where all the surface species are immobile except those on the limited active part of the surface during the reaction.