Compressional and shear wave velocities have been measured as functions of pressure to 3 GPa for a suite of low-porosity, isotropic, polycrystalline germanate and silicate spinels using the phase-comparison variant of ultrasonic pulse interferometry. Curvature in velocity-pressure trends below a threshold pressure of order 1.5 GPa is caused by closure of microcracks and imperfect acoustic coupling between the specimen and the acoustic buffer rod. Pressure derivatives of wave velocity are determined from linear velocity-pressure trends above this threshold pressure. Previous work on sintered polycrystals of comparable acoustic quality has demonstrated that pressure derivatives determined in this way match those of the ideal zero-porosity aggregate within 10%. The measured derivatives are used to calculate the pressure derivaties dK/dP and dG/dP of the bulk and shear moduli. Although systematic variation of dK/dP with composition is not evident, variation of dG/dP with K/G within the silicate and germanate series suggests a likely value of dG/dP~1.9 for &ggr;-Mg2SiO4. ¿ American Geophysical Union 1991