Upon oxidation in acetonitrile, pyrrolidone analogues, namely pyrrolidone, methyl-pyrrolidone, ethyl-pyrrolidone and N-vinyl pyrrolidone, afford thin organic films which can be analyzed by X-ray photoelectron spectroscopy. One common singular feature is noted in the XPS spectra: the advent of a very high energy Cls peak (288.9 eV), indicating the formation of a molecular structure with a carbon atom in an unusually electronegative environment. Considering the recently proposed reaction mechanisms for the oxidation of pyrrolidone analogues, a theoretical study is carried out: several molecular structures are proposed, on which very accurate calculations of core-electron binding energies are performed using the density functional theory. On comparison with the complete experimental spectrum, it is proposed that this peculiar high-energy peak stems from the opening of the pyrrolidone ring to yield carboxylic acid groups.