Steady-state and time-resolved fluorescence quenching techniques were used to study the interaction of short-chain N-alkylpyridinium ions with sodium dodecyl sulfate (SDS) micelles using pyrene as a probe. The rate constants of the quenching of pyrene and binding constants of the cations to SDS micelles were obtained in different conditions of surfactant and salt concentrations. The increase in the number of carbon atoms in the alkyl chain (ethyl, propyl, butyl, and hexyl) of these counterions leads to a reduction of both intramicellar mobility and exchange rate with the aqueous phase. The intramicellar quenching rate constant value is in the range of (3-6) x 10(7) s(-1) while the exit rate constant of the organic cations decreased and is in the range of (1-8) x 10(5) s(-1). The fluorescence decay parameters of pyrene in the presence of ethyl, NEP+, and propyl, NPP+, pyridinium cations were treated based on the ion-exchange formalism. The selectivity constants obtained, 13 +/- 1 and 47 +/- 1 for NEP+/Na+ and NPP+/Na+, respectively, suggest that the highest value for NPP+ cation is ascribed to electrostatic field contribution in addition to a specific adsorption potential due to the size of the hydrophobic chain, leading to a reduction of the alkyl chain-water contacts of both organic cation and surfactant monomer. These results show that the selectivity was determined by the nature of the counterions present and it is less dependent on the characteristics of the micellar aggregates.