Ion exchange equilibria of alkali metal ions (Li+, Na+, K+,, Rb+, and Cs+)H+, systems have been studied in MNO-j-HNOj media with ionic strength of 0.1 at 30, 45 and 60 °C on tin(IV) antimonate as a cation exchanger. The ion exchange isotherms have been measured for both forward and backward reactions by the batch technique. The isotherms showed S-shaped curves for all exchange systems studied. The selectivity coefficients (logarithmic scale) vary with the equivalent fraction XMof alkali metal ions in the exchanger and give two linear functions of XMwith a break point (XM= 0.14, except 0.04 for Li+, /H+) indicating two different exchanging sites. The selectivity sequence, Na+, ⩾ K+, ⩾ Rb+, ⩾ Cs+, ⩾ ⩾ Li, holds in the range of Xu= (0 - 0.04) and the sequence, Cs < Rb+, ⩾ K+, ⩾ Na+, < Li+, applies when XMis higher than 0.14.