The investigation of the acidic equilibrium of 5(p-dimethylaminobenzylidene)rhodanine (PDR) in aqueous micellar solution by a simple spectrophotometric method is reported. For this purpose, the effect of nonionic (brij-35 and TritonX-100), cationic (CTAB), and anionic (SDS) surfactants on the absorption spectra of PDR at different pH values was studied. The pK(a) of PDR in a different concentration of each surfactant was determined (27 degreesC). It was found that PDR is more dissociated in the presence of CTAB and the lowest dissociation is observed in SDS micellar media. These changes in pK(a), can be explained according to the electrostatic contribution of binding of the anionic form of PDR to the micellar pseudophase.