The interaction of the Cr3 and V2 impurity ions with the alpha - Al2O3 crystal lattice was studied by observing the hydrostatic pressure dependence of their electron paramagnetic resonance spectra to 6000 atmospheres. A two sample comparison method was used in which two identical samples were placed in the X band microwave resonant cavity. One end of the cavity and one sample were subject to hydrostatic pressure. The changes in the spin Hamiltonian parameters were obtained by measuring the difference in the magnetic field required for the same resonant condition in each sample as a function of pressure. From this it was concluded that the ions occupy similar lattice sites, though possibly including the displacements proposed in the literature, and are subject to the same compressibility, comparable to that of the bulk compressibility. Comparison with the point charge model led to the conclusion that the point charge model was inadequate to predict the observed pressure changes. A molecular orbital calculation as a function of surrounding ion position and further experiments are suggested.