The structure and copper valence states of the 100K superconductor, Ba 2YCu 3O 7 have recently been determined by neutron powder diffraction between 5K and 300K 1, and at room temperature 2. This ′oxygen deficient orthorhombic perovskite structure′, now independently confirmed 3,4, is well ordered and stochiometric, and contains one Cu +++ and two Cu ++ atoms distributed over two square planar oxygen co-ordinated sites. In the a-b plane CuO 4 squares are linked by their oxygen corners to form infinite sheets, while along the b-axis CuO 4 squares form infinite chains on the second Cu-site. In this paper we show that when Ba 2YCu 3O 7 is heated above room temperature, it progressively loses all oxygen from the O4 sites on the b-axis chains, while the remaining oxygen sites remain fully occupied. Near 700°C it becomes tetragonal 5, with the P4/mmm Ba 2YCu 3O 6 structure obtained by X-ray diffraction at room temperature 6. This material is well ordered, stochiometric and stable when cooled under vacuum. The valence of Cu2 in the a-b sheets remains close to 2, while that of Cu1 in the b-axis chains falls to near 1. The valence of copper in these chains can perhaps fluctuate, since the vibration of O4 out of the chain, which we observed even at 5K 1, implies a reduction of the valence charge on this copper site. We have observed similar large amplitude oxygen vibration at low temperature in La 2CuO 4.