The geometric and the electronic structures of terbium-silicon anions, TbSin-(6 less than or equal to n less than or equal to 16) were investigated by using photoelectron spectroscopy (PES) and a chemical-probe method. The clusters were produced by a double-rod laser vaporization technique. From trends observed in the electron affinities (EAs), the TbSin- clusters were categorized into three groups of (I) 6 less than or equal to n less than or equal to 9, (II) n = 10, 11, and (III) n greater than or equal to 12. Together with adsorption reactivity toward H2O it is concluded that a Tb atom is encapsulated inside a Si-n cage at n 2: 10; Tb@Si-n.