The temperature dependent structural phase transition from the tetragonal chalcopyrite like structure to the cubic sphalerite like structure in CuInSe 2 was investigated by in-situ high temperature synchrotron radiation X-ray diffraction. The data were collected in 1K steps during heating and cooling cycles (rate 38 K/h). The Rietveld analysis of the diffractograms led us to determine the temperature dependence of the lattice parameters, including the tetragonal deformation, I1-ηI, and distortion Iu-¼I (η=c/2a, a and c are the tetragonal lattice constant; u is the anion x-coordinate). The thermal expansion coefficients α a and α c of the tetragonal lattice constant which are related to the linear thermal expansion coefficient α L were obtained, as were α a of the cubic lattice constant, also α u and α η . The transition temperature is clearly identified via a strong anomaly in α L . The temperature dependence of the anion position parameter was found to be rather weak, nearly α u ~0, whereas α η increases slightly. However, both increase strongly when approaching to within 10 K of the transition temperature (the critical region) and I1-ηI as well as Iu-¼I go to zero with IT-T trans I 0.2 approaching the phase transition. The cation occupancy values, derived from the Rietveld analysis, remain constant below the critical region. Close to the transition temperature, the number of electrons at the Cu site increases with a dercrease in the number of electrons at the In site with increasing temperature, indicating a Cu-In anti site occupancy, which is assumed to be the driving force of the phase transition. At the transition temperature 67% of Cu + were found to occupy the Me1 site with a corresponding 67% of In 3+ at the Me2 site. Although full disorder is reached with 50%, this level seems to be high enough that the phase transition takes place. The order parameter of the phase transition, goes with IT-T trans I β to zero with the critical exponent β=0.35(7) which is in good agreement to the critical exponent β=0.332 calculated for order-disorder transitions according to the Ising model.