A pronounced odd-even alternation for Al－n clusters ranging from n=5 to n=17 has been observed in mass abundance spectra. The electron-pairing effect of the delocalized valence electrons determines intrinsically the alternation, while special source conditions enhance the alternation so dramaticaly that it is readily obseervable in mass abundance spectra. The ion group Al7O－ has also, for the first time as far as we know, been observed, and its geometric as well as electronic structure has been discussed.