Association constants of alkylpyridine bromide, CnPyB, (methyl, ethyl, propyl, and butyl) to dodecylsulfate micelles has been determined by two methods, ultrafiltration and fluorescence spectroscopy. In the last, the emission intensities of micelle-solubilized probe (pyrene) were measured in the absence and presence of a quencher (CnPyB) distributed between the aqueous and the micellar phase. The Gibbs energy of transfer of CnPyB allows the separation of the contributions due to CH2 and pyridinium groups. From the association of 4-ethylpyridine at pH 3 and at pH 6, it is possible to determine the contribution to the Gibbs energy (u(E1)) due to the Coulombic interaction between a formal charge on the substrate and the charged micellar surface. Results are related to the position of the substrates on the micellar surface.