The interactions between polypyrrole and an embedded bromophenol blue (BPB) dye at different oxidation levels and pHs were investigated. Cyclic Voltammetry, in situ UV-vis and Raman spectro-electrochemistry were used to detect electrochemical and optical changes in the BPB-doped ppy films. The acid-base equilibrium of the embedded dye was observed to be highly dependent on the applied potential and on the pH conditions. The intensities of the vibrational bands characteristic of the acidic and basic forms of the species inside the film clearly showed a potential dependent shift of the dissociation equilibrium of BPB, which is effectively controlled by the oxidation level of ppy. This type of information provides some insights into the effect of the environment on the chemical behavior of acid-base pairs embedded in polymeric matrixes. (C) 2011 Elsevier Ltd. All rights reserved.