A new technique using ``amorphous'' boron as a pressure vessel yields satisfactory Debye—Scherrer patterns for substances subjected to quasi‐hydrostatic pressures ranging up to at least 150 kbar. The pressure attained depends on sample size and its compressibility. In general, the pressure attained is higher for the more incompressible substances. The following substances have been used to test the apparatus: Fe, NaCl, KCl, AgCl, CaCO3, and NaNO3. Evidence for polymorphic transitions was found for all substances except NaCl. The changes in lattice spacings and the observed transformations are consistent with the transition pressures and volume decrements determined by Bridgman or by shock‐wave experiments in the case of Fe. The high‐pressure structures are tentatively identified.