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X-ray investigations of Pr2CuO4 and La2CuO4 under pressure have been carried out at 300 K and 30 K. The lattice parameters decrease monotonically for both structures, thus preserving the structure of the materials. For La2CuO4 the lattice parameter reduction is isotropic with increasing pressure, but the lattice parameters of Pr2CuO4 are found to behave anisotropic, as the c-axis decreases faster than the a-axis. La2CuO4 has a bulk modulus of 1850 kbar at 300 K, whereas Pr2CuO4 has a bulk modulus of 1600 kbar at 300 K and 1650 kbar at 30 K, due to the faster decreasing c-axis lattice parameter. In the a and b direction both materials have a similar compressibility. Assigning this behaviour to the common copper-oxide layers of both structures gives the possibility to generalize these results. For several high temperature superconductors we estimated the linear compressibilities in the a, b direction being in the range of 1.5 to 2.0×10-4 kbar-1. In the c-direction the compressibility apparently depends on the degeneracy of the oxygen octahedra and the number of intercalated layers orstructural elements. © 1989.