Abstract A behavior of polyaniline (PANI) composites with common polymers under different conditions is often a result of a synergetic combination of properties of their components. In this work we consider specificity of such a combination in terms of both hydrogen bonding of PANI with a matrix polymer poly(ethylene terephthalate) (PET) and its influence on interactions of the composite PANI–HCl/PET films or dedoped PANI/PET films with alkaline and alkali metal halide aqueous solutions. In particular, it is found that PANI in emeraldine base (EB) state forms a EB complex with hydroxide anions, which is confirmed not only by a strong blue shift of the exciton band in UV–vis spectra of both the dedoped PANI/PET (up to 529 nm) and pure EB film (up to 579 nm) but also by reversibility of this shift and its independence on the alkali cation nature. Additional confirmations of existence of this complex were obtained by quenching of the PL emission of EB by OH − anions in the dedoped PANI/PET and IR spectra of EB film after its interaction with the alkali solution. Formation of this EB–OH − complex is shown to be due to hydrogen bonding of hydroxide anions with amine groups of the dedoped PANI (EB) backbone. The EB–OH − complex is much more stable in the dedoped PANI/PET composite film than in the pure EB film probably due to additional hydrogen bonding of OH − anions with carbonyl oxygen atoms of the matrix. However, this complex in the PANI/PET film can be destroyed by dissolving in NMP or by contact with the solutions of alkali metal halides. Specifically, the complex instability in these solutions is enhanced in the halide anions range F − − − that results in restoring the EB exciton band position and IR absorption bands of the pure EB.