The blue thin polyaniline base film changes its color to green after immersion of the film into dibutyl phosphonate. The green color of the film converts to a greenish-blue after heating to 200 degrees C in air, which is characteristic for the protonated conducting form of polyaniline. This is in contrast to the "standard" polyaniline hydrochloride, which is transformed into a cross-linked polyaniline base under such conditions. To explain this unexpected observation, the interaction of polyaniline base with dibutyl phosphonate at ambient conditions and after heating to 200 degrees C was studied using UV-visible, FTIR and Raman spectroscopies. On the basis of these studies, we propose that the dibutyl phosphite tautomeric form of dibutyl phosphonate, which interacts with polyaniline base at 20 degrees C, converts to the oxidized form, dibutyl phosphate, at 200 degrees C and subsequently protonates the film. Quantum-chemical modeling of the interaction of polyaniline base with dibutyl phosphite and dibutyl phosphate supports this explanation.