series of HIP cycles were made on powder samples of the high Tc superconductor YBa2Cu3O7-x in the range of 600 to 1000 C, and pressures of 150 to 200 MPa. The high Tc superconducting orthorhombic phase is difficult to fully densify, because the material easily decomposes and releases oxygen. Two types of encapsulation materials have been used, mild steel and borosilicate glass, of which, glass has a lower oxygen diffusivity and reactivity than steel. Two decomposition regimes were observed. The first regime is from about 600 to 850 C when there is a partial decomposition leading to oxygen backpressure in glass and a transformation of the orthorhombic phase into the tetragonal phase in steel. The second stage of decomposition is above about 900 C, when the superconductor powder in a steel encapsulation decomposes into simpler oxides, and in glass nearly full density results, but without the desired orthorhombic YBa2Cu3O7 structure. Addition of excess CuO has been found to yield the 124 phase (YBa2Cu4O8) superconductor. The 124 phase is the thermodynamically favoured phase at high temperatures and pressures. The 124 phase samples reached nearly full density at the highest temperatures used