Scanning electrochemical microscopy surface interrogation (SI-SECM) in the cyclic voltammetry mode was successfully used to detect and quantify adsorbed CO on a Pt electrode by reaction with electrogenerated Br-2. The two-electrode setup used in this new technique allowed the production of Br-2 on an interrogator tip, which reported a transient positive feedback above a Pt substrate at open circuit as an indication of the reactivity of this halogen with CO(ads). Br-2 and CO2 are shown to be the main products of the reaction (in the absence of O-2), which may involve the formation of bromophosgene as a hydrolyzable intermediate. Under saturation conditions, CO(ads) was reproducibly quantified at the polycrystalline Pt surface with theta(co) approximate to 0.5. The reaction is shown to be blocked by the action of pre-adsorbed cyanide, which demonstrates the surface character of the process. The formation of CO2 as an end product was further tested in a bulk experiment: addition of Pt black to a mixture of Br-2 in 0.5 M H2SO4 through which CO was bubbled gave a precipitate of BaCO3 in a saturated solution of Ba(OH)(2). The use of SI-SECM allowed access to a reaction that would otherwise be difficult to prove through conventional electrochemistry on a single electrode.