Evaluation of numerous data from different methods of measurement and theories applied in high-pressure physics shows that the strain energy of a monophase system, which represents the low- or a high-pressure phase of a hydrostatically compressed substance mostly relevant to planetary physics, approximates to a term proportional to the square of an auxiliary function of the volumetric contraction, the bulk modulus and its successive partial derivatives with respect to pressure at the initial state of the thermodynamical system. From this it follows that new-defined polynomial equations in the initial values of the bulk modulus and its pressure derivatives must be satisfied, which is checked for many elements, halides, oxides, minerals, and rocks. Using a suitably chosen auxiliary function, the strain energy, pressure, bulk modulus and its first pressure derivative at any equilibrium state of the monophase system are represented by approximation functions of the volumetric contraction as well as the initial values of the bulk modulus and its derivatives. The information content of these functional relations called Model N if they include derivatives of the bulk modulus only up to the order N surpasses, already in the cases N = 1 end N = 2, that of other relations hitherto used for interpreting the compressional behaviour of different substances. Model 1 is discussed on the base of more than 70 substances with various compressional properties, among them stishovite and solid hydrogene, proved to be suitable, in particular, for the physics of the interiors of the Earth and of terrestrial planets. Considering the reliability of the data material available, Model 1 is pointed out to predict experimental values from the tested substances for volumetric contractions down to about 0.5 with a relative error between 1 % and 5 %. Model 2, extending the information volume of Model 1, turns out to be appropriate for volumetric contractions less than 0.5 as is the case in the interiors of the Jovian planets. For practical use of both models tables and graphs of sensitivities and errors of significant quantities are presented. In case data from shock-wave experiments on rocks and minerals are available, Model 1 is used for identification of crystal structures of high-pressure phases. As a result, for the majority of high-pressure phases the compressional behaviour differs from that of the corresponding oxide mixtures. For some substances new conclusions on their crystal structures can be drawn.