The electronic structures of Yb 2 Co 3 X 9 (X=Ga, Al), which show properties of heavy-fermion antiferromagnet for X=Al and mixed-valent behavior for X=Ga, are studied by means of high-resolution photoemission spectroscopy. A sharp Yb 4f related peak, which shows characteristics consistent with the prediction by the single impurity Anderson model, was observed just below E F only for X=Ga. The origin of the difference between the Ga-based and Al-based compounds is attributed to the difference in the 4f energy level and the 4f-ligand hybridization strength.