Mixed metal clusters of sodium with copper, silver, and gold were produced by supersonic expansions at well-defined thermodynamic conditions. Differences in the behavior toward binary alloying, cluster formation, and relative thermodynamic stabilities are discussed within a structureless, delocalized, electronic shell model and using molecular orbital arguments and the different strengths of the relativistic effects of the coinage metals. Free-energy changes (Delta G) of selected exchange reactions are determined, showing the substitution of gold atoms to be thermodynamically the most favored. This is due to the preferred s-d hybridization in gold atoms, leading to a stronger metal-metal interaction. The measured ionization potentials reveal a distinct shell closing at the octamer for all three heterosystems. The drop of the ionization potential. going from the octamer to the nonamer is most pronounced for the sodium-silver heteroclusters and is almost as large as for pure sodium clusters.