Vapor nucleation induced by seed ions of controlled size and charge is studied for several monovalent positive and negative ions as well as divalent cations. Ions from dissolved salts are transferred into a gas via electrospray, purified by differential mobility selection, and introduced into a condensation nucleus counter of the mixing type using supersaturated dibutyl phthalate vapors. The number of drops nucleated on the ions is then measured as a function of the vapor supersaturation S. None of the anions tested is activated. Singly charged cations with mobilities Z from 0.48 up to 0.93 cm(2)/V/s are activated at values of ln S 30% smaller than predicted by Thomson’s model. All doubly charged cations tested (Z from 0.46 to 1.08 cm(2)/V/s) fall very near the Thomson curve, independently of their size or of whether the double ionic charge is localized on one metal atom or on two separate monovalent sites.