The effect of Cs atom doping on metallic aluminum clusters and covalent silicon clusters, Al n Cs m (n=5–14, m=0–3) and Si n Cs m (n=5–16, m=0–3), was examined by mass spectrometry and anion photoelectron spectroscopy. For clusters containing Cs atom(s), the electron affinities of both clusters are generally decreased and the following characteristic features are observed: for Al n Cs m , Cs-atom doping causes (1) electron filling into the electronic shell structure of the Al n clusters and (2) geometrical packing of icosahedral 13-atoms, while for Si n Cs m Cs-atom doping enhances electronic stability to be ascribed to pure Si n clusters, particularly at (n,m)=(10,3) and (13,1).