The insertion of alkali metals in chalcopyrite has been investigated by X-ray photoelectron spectroscopy and band structure calculations of the extended Huckel type. The results show that the alkali metal insertion (removal) reaction, which corresponds to the discharge (charge) process of the electrochemical system Li/Li1+ / CuFeS2, is related to an electron transfer involving mainly the Fe ions. The experimental and theoretical data indicate that the Cu atoms do not participate in the redox reaction.