We investigated the chemical states of In3Sb1Te2 (IST) thin film using high-resolution X-ray photoelectron spectroscopy (HRXPS) with the synchrotron radiation during in-situ annealing in ultra-high vacuum. To obtain the oxygen-free amorphous IST (a-IST), we performed the mild Ne+ ion sputtering. And also we confirmed the relative a-IST stoichiometry to be 54%:17%:29% based on HRXPS data. At the first and second phase transition temperatures of 350 and 400 degrees C, we observed the dramatic changes of chemical states from a-IST to InSb and the mixture of crystalline-IST and InTe, respectively. There was a depletion of Sb atoms on the surface after annealing at 750 degrees C. We assume that Sb atom is a key for the phase transition in IST. However, chemical state of the Sb in IST is unstable during the phase transition and it will be caused with the non-reversible process by this structural instability. (C) 2013 Elsevier B.V. All rights reserved.