The formation and decomposition processes of the Ca 0.5Sr 0.5CuO 2 high pressure phase with the so-called infinite-layer structure have been followed in-situ by energy dispersive X-ray diffraction using a synchrotron radiation source. The decomposition reaction of the infinite-layer phase is produced at 1200 °C to yield a mixture of phases including the n= 2 member of the Sr n−1Cu nO 2n−1 family and (Ca, Sr) 2CuO 3. This reaction implies a volume reduction and the most significant changes are observed in the a–b plane implying that the CuO 2 planes are partially condensed.