We report experimental values for the band-edge hydrostatic deformation potentials of GaAs and InP semiconductors. The deformation potentials are determined directly by observation of the universal pressure derivative of transition-metal defect levels and consideration of this pressure derivative in the context of recent models of heterojunction band lineup. We obtain values of -9.3 and -7.0 eV for the conduction-band deformation potentials of GaAs and InP, respectively, determined from the uniaxial-stress deep-level transient spectroscopy on defect levels of Ti and V.