We have studied the salt effect on the clustering of a nucleoside (cytidine) in water by means of an electrospray mass spectrometer, which is specially designed for the analysis of clusters isolated from solution. In comparing the formation of cytidine clusters in the presence of NaCl, KCl, or MgCl2, Mg2+ promoted the formation of cytidine clusters until octamer, whereas the effects of Na+ and K+ on the cytidine clustering was not as efficient as that of Mg2+. It was also demonstrated that the observed salt effect on the cytidine clustering is related to the structures of observed hydrated ions, that is, Mg2+ formed its hydrated clusters with less than 12 water molecules, whereas Na+ and K+ with less than 6 water molecules.