Interactions between alkali-metal cations and silicate anions in silicate solutions were investigated by cation NMR spectroscopy. The concentration of cation-anion pairs was determined from the chemical shift and the resonance line widths. The concentration of ion pairs involving large anions increases with increasing cation size. For silicate solutions in which there is a distribution in cation size, the concentration of ion pairs exhibits a maximum with increasing cation size. This maximum is interpreted in terms of the selectivity of ion-pairing reactions.