The method of photochemical hole burning allows one to study extremely small matrix rearrangements by investigating strain-induced shifts and broadenings of quasi-homogeneous optical lines of dye molecules in amorphous matrices. Both organic (polyethylene and polystyrene) and inorganic (Vycor glass) matrices were doped with free-base phthalocyanine and investigated under hydrostatic pressure conditions. The experimental data confirm theoretical predictions about the frequency dependence of the pressure-induced shift and broadening behaviour of spectral holes. The underlying theoretical assumption about the interaction between solute and solvent molecules and about the structure of the glassy system can be tested by comparing the experimental data and theory. In this way, information on the microscopic behaviour of amorphous systems under pressure variations can be obtained. The optically determined compressibility values agree with the mechanically measured values within 10 – 15%.