Abstract A model to determine ion exchange constants, Kex, in polyelectrolytes through interfaces is proposed. The model is tested through the chloroform/water interface using the bromide salts of n-alkylpyridinium polyelectrolytes (n=6, 8, 10) in the organic phase and the sodium salts of benzene sulfonate, 4-methylbenzene sulfonate and 4-ethylbenzene sulfonate in the aqueous phase. The results show good agreement with the initial linearity predicted by the model, but a limit of saturation is reached as the bromide anions are exchanged by the amphipathic counterions of increasing size. The ion exchange constants increase with the size of the sulfonate counterions and with the length of the polyelectrolyte side chain. Benzene sulfonate and bromide ions show similar affinity for the polyelectrolyte, as inferred from the values of Kex exchange constants. The results are in good agreement with the reported Kex values in a micellar system with a trimethyl ammonium polar head when compared with extrapolated Kex for the same type of ion exchange.