The crystal size of nanocrystalline CdS and CdSe films, electrodeposited from dimethyl sulfoxide solutions containing a Cd salt and elemental S or Se, is shown to depend on the nature of the anion of the Cd salt, Relatively strongly adsorbing anions, such as chloride, result in a smaller nanocrystal size than relatively nonadsorbing anions such as perchlorate. This difference in nanocrystal size is explained by blocking (or capping) of the growing crystals by the adsorbed ions. Very strongly adsorbing species, such as alkyl phosphines, result in even smaller crystal size (3.5 nm average diameter).