A new model has been developed to describe the mechanism which determines the preserving or reversing of the phase transformation for diamond and hard BN (boron nitride) during a shock wave experiment. The model considers the principal distribution of parameters, such as stresses, temperature, mass velocity, and density both within these materials, and in their mixtures with other materials. Four main stages in the shock wave experiment are identified to reflect the qualitative changes in these parameters. The possibility of preserving synthesized hard phases of carbon and BN is shown to depend primarily on the characteristics of the release process after maximum pressures are achieved. If the release process is almost adiabatic, then the transformation of strain energy into heat and powerful tensile stresses causes graphitization. If the release process is almost isentropic, then conditions can be favorable for preservation of the hard phases. 0 I995 American Institute of Physics.