Geometric and electronic properties of planar configurations of Cu-Ni clusters containing up to five atoms are studied using density functional theory with effective core potentials, focusing on the differences between mono and bimetallic clusters. Reactivity is examined by testing hydrogen adsorption on bridge and on-top sites. It is found that geometric effects have strong effect on reactivity; however, the significant differences between the Cu and Ni atomic electronic structures are strong factors on the nucleation. stability, and adsorption properties.