The physicochemical interactions of thermally evaporated magnesium atoms with the chemically synthesized polypyrrole film in its salt and partially undoped (deprotonated) forms were studied in situ by x-ray photoelectron spectroscopy (XPS). The changes in the N(Is), Mg(2p), and S(2p) core-level spectra of the films, as well as the changes in the interfacial chemical compositions, were carefully monitored. It was observed that both types of films underwent an increase in intrinsic oxidation state ([=N-]/[-NH-] ratio) in response to the metal deposition process.. Careful analysis of the Mg(2p) and S(2p) spectra revealed that the incoming magnesium atoms reacted with the dopant to form a salt. The observed increase in the intrinsic oxidation state of the films could therefore be attributed to the undoping (deprotonation) process. The interactions had also involved oxygen since a surge in oxygen concentration at the metal/polymer interface was observed with increasing magnesium coverage. From the results obtained, it could be concluded that the interfacial layer consisted of magnesium-dopant salts, magnesium oxides, magnesium cluster, and magnesium metal.