The formation of alloy clusters using a plasma-gas-aggregation technique is described for Co-Al and Co-Pt systems. This method employs two separate elemental sputtering sources and a growth chamber. Metallic vapors generated were cooled rapidly in an Ar atmosphere, and grown into alloy clusters. The composition of the clusters was controlled by adjusting the ratio of the applied sputtering power. We found that B2-CoAl clusters of about 12 nm in diameter were formed for a composition range wider than that predicted by the Co-Al phase diagram, and that high-temperature fcc-CoPt clusters were formed in the Co-Pt system. These findings suggest the nonequilibrium nature of the cluster formation. The size distribution of the clusters is highly monodispersive and does not follow commonly observed log-normal distribution. These results were discussed from the viewpoint of simple gas dynamics. We concluded that monomer absorption with discrete residence time is the dominant mechanism for monodispersive alloy cluster formation, and that the contrasting thermodynamical features between the Co-Al and Co-Pt systems are at the cause of the observed difference in average cluster size.