To elucidate how surface and gaseous phases interact each other to induce chemical reactions, X-ray photoelectron spectroscopy (XPS) analyses were carried out on powders as milling took place. An acute analysis of data acquired by the XPS-technique allowed us to find a series of well-defined chemical transitions from precursors to the stoichiometric PbTe phase. By coupling, theoretical and experimental data a self-consistent model was developed. Initially, the process manifested itself as an oxidation stage and secondly as a reducing process. In agreement with a thermodynamic evaluation of free energy of phases traced during milling, chemical transitions were traced as Te2+ to TO in oxidation reactions. If high oxygen potential prevails in the milling system subsequently Pb2+ evolves to Pb4+. On the other way, high valence oxides like Pb4+ or Te4+ were reduced to Pb2+. and Te2-. However, the last transition an asymmetric transformations was identified as non-stoichiometric phases. (C) 2012 Elsevier Ltd. All rights reserved.