Adsorption of common molecules over two-dimensional Au9, Pt9, and Au18Pt18 is investigated with implementation of first-principles calculations. In general, it is found that Pt9 and Au18Pt18 exhibit low adsorption energies where Au18Pt18 preserves the structural integrity of the molecule and surface. In particular, adsorption of molecules onto Au18Pt18 frequently results in low adsorption energies and high reactivity with minor surface reconstruction of Au18Pt18 and average bond lengths of molecules. The decrease in adsorption energy can be attributed to the presence of platinum, while gold can be considered responsible for structural stability. In addition, molecule dissociation is observed in the cases of H2, HCl, CH4, SO, and SO2 when Pt atoms are involved. Thus, two-dimensional Au9, Pt9, and Au18Pt18 show low adsorption energies against common molecules, reflecting adsorption energies observed in small Au and Pt clusters. These results demonstrate that Au18Pt18 can successfully utilize the low adsorption energies associated with platinum while preserving the integrity of the surface structure using gold atoms, making it possible to adsorb desired molecules using select areas of the Au18Pt18 surface.