The ion exchange behaviour of pyridinium molybdoarsenate has been investigated with some uni-and bi-valent cations. The slopes of the isotherms are widely different for various cations suggesting that separation of these ions is feasible. Barrer's equation has been applied to the exchange reaction and various thermodynamic parameters, viz. interaction energy between the ions on surface, thermodynamicequilibrium constant and free energy change for the exchange process, have been evaluated. This exchanger is found to be highly specific for TI+ ions and can be used for its removal from mixtures of various uni-and bi-valent cations.