We have studied geometric and electronic structures of metal (M) atom doped silicon (Si) clusters, MSin (M = Ti, Hf, Mo and W), using mass spectrometry, a chemical-probe method and photoelectron spectroscopy. In the mass spectra for all of the mixed cluster anions, MSin-, both MSi15- and MSi16- were abundantly produced compared to neighbors. n greater than or equal to 15. Together with the result of the adsorption reactivity and photoelectron spectroscopy, it has been revealed that one metal atom can be encapsulated inside a Sin cage at n greater than or equal to 15. (C) 2003 Elsevier Science B.V. All rights reserved.