The optical properties of small Cu-Ag bimetallic clusters have been experimentally and theoretically investigated in relation to their chemical structure analyzed by high resolution transmission electron microscopy (HRTEM). Cu (1-x)Agx clusters of about 5 nm in diameter are produced in a laser vaporization source with a well-defined stoichiometry (x = 0, 25, 50, 75, and 100%) and dispersed in an alumina matrix. Absorption spectra are dominated by a broad and strong surface plasmon resonance whose shape and location are dependent on both cluster composition and sample aging. Detailed modeling and systematic calculations of the optical response of pure and oxidized mixed clusters of various chemical structures have been carried out in the framework of classical and semiquantal formalisms. Optical and HRTEM measurements combined with theoretical predictions lead to the conclusion that these bimetallic clusters are not alloyed at the atomic scale but rather present a segregation of chemical phases. Most likely, they adopt a Cu@Ag core-shell configuration. Moreover, the nanoparticle oxidation process is consistent with the formation of a copper oxide layer by dragging out inner copper atoms to the cluster surface. © 2015 American Chemical Society.