The excitation spectra associated with the group-III acceptor impurities; boron, aluminum, gallium, and indium in silicon, have been studied at low temperatures using grating spectrometers. The behavior of these spectra under uniaxial stress has been studied employing polarized radiation. The improved resolution of the present investigation has revealed details not reported previously for the zero-stress spectra. The line reported by earlier workers at 68.26 meV for gallium impurity has been resolved into three components at 67.95, 68.25, and 68.43 meV. For aluminum and indium impurity, the lines at 64.99 and 150.88 meV, respectively, have each been resolved into two components, at 64.96 and 65.16 meV and at 150.80 and 151.08 meV, respectively. The presence of the shoulder at 39.91 meV on the excitation line of boron at 39.64 meV has been confirmed. Also, transitions from the ground state associated with the p32 valence band to excited states associated with the split-off p12 valence band have been observed for gallium and indium impurity, in addition to those of boron and aluminum reported previously. It has been established that the ground state has Γ8 symmetry for all the impurities studied. The first three excited states participating in the optical transitions (corresponding to lines 1-3) also have Γ8 symmetry. The behavior of the line-4 complex under stress is consistent with the final states being Γ6+Γ7+Γ8. The stress-induced sublevels of the Γ8 final states of lines 1 and 2 have opposite ordering for compression along 111. The various states of a given impurity differ in their quantitative response to stress. Also, there are marked differences in the quantitative stress behavior of these states from impurity to impurity. © 1967 The American Physical Society.