The pyridine-2-azo-p-dimethylaniline (PADA) ligand presents two acid dissociation constants, being pK(a1) related to the pyridinium and pK(a2) related to the anilinium residue. These have been measured by spectrophotometric titrations in aqueous solutions containing either the anionic (SDS), or the nonionic (Triton X-100) or the cationic (DTAC) surfactants. The pK(ai) shifts of the charged systems from that of the PADA/Triton X-100 reference (Delta pK(ai)(0)) are compared. For PADA/DTAC Delta pK(a1)(0) = 0.05 and Delta pK(a2)(0) = 0.6. For PADA/SDS Delta pK(a1)(0) = 2.1 and Delta pK(a2)(0) = 2.1 both yielding the value of -126 mV for the surface potential (psi)of SDS. The psi value, lying between the calculated Stern potential and the zeta potential, indicates that the dye is located on the SDS micelles between the fixed and the shear layer. In contrast, the behaviour of PADA/DTAC is explained assuming that the positively charged deprotonation sites of PADA are forced to protrude towards the bulk solvent by the positive charges of DTAC micelles. The shifts of the apparent pKai from the aqueous values (Delta pK(ai)(W)) have also been analysed. Concerning PADA/Triton X-100, the shifts Delta pK(a1)(W) = -0.1 and Delta pK(a2)(W) = -0.9 are rationalized in terms of dielectric constant reduction at the reaction sites. Concerning PADA/DTAC, Delta pK(a1)(W) = -0.05 and Delta pK(a2)(W) = -0.3 whereas, for PADA/SDS, Delta pK(a1)(W) = 2.0 and Delta pK(a2)(W) = 1.2. The pK(a2)(W) values decrease on raising the surfactant concentrations for all the investigated systems. This behaviour is explained assuming that the increase of the overall micellar surface and, by consequence, of the reaction sites number, results in a site dilution effect which disfavours proton association. The addition of NaCl induces changes of pK(a1) and pK(a2) which are explained in terms of (large) reduction of psi for PADA/SDS and of (small) reduction of the dielectric constant for the other systems. (C) 2014 Published by Elsevier B.V.