Engineering Chemistry Laboratory, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh Manuscript received 29 March 1975; revised 17 July 1975; accepted 8 August 1975 Aluminium-nicotine exchange equilibria in dilute suspensions of illite were studied at 25° and 40°, Exchange isotherms were drawn and thermodynamic equilibrium constants and the standard free energies of exchange were evaluated from the selectivity coefficients. The results indicated a preference of nicotine by the clay system at both the temepratures upto a limited concentration. The negative enthalpy effects and entropy loss accompanying the reaction showed a stronger and more ordered binding of the nicotinium ions on the clay surface, Values of surface phase activity coefficients of the adsorbed ions and the excess thermodynamic functions indicated that the equilibria involving aluminium and nicotine behaved as a non-ideal system.