The surface diffusion (spillover) through the intermetallic Pt/Pd junction of adsorbed CO (COad) during the formic acid electrooxidation reaction (FAO) was analyzed on a bimetallic Pt/Pd electrode by chronoamperometry followed by stripping voltammetry. An ordered matrix of equally spaced square palladium islands on a platinum substrate was prepared by the Mask-Assisted E-Beam (MAEB) method in order to have a large interface length/area relationship. Potential steps were applied in 0.5 M HCOOH + 0.5 M H2SO4 solution (0.2 V <= E-s vs. RHE <= 0.5 V) during times t <= 500 s in a flow cell. After changing solution to 0.5 M H2SO4, the amount of COad on Pt and Pd were measured by voltammetric stripping. Under these experimental conditions, the pure metals show the presence of COad on Pt but not on Pd. The results obtained on the bimetallic electrode demonstrate the COad spillover from Pt towards Pd through the heterometallic junction.