Complete high‐pressure single‐crystal structure determinations have been performed up to 90 kbar on ruby (Al2O3with less than 0.05 wt&percent; Cr2O3) using an automatic diffractometer. The rhombohedral lattice constantaRis found to decrease linearly within the given error limits fromaR=5.136±0.004 toaR=5.075±0.005 A˚ at 90 kbar. The rhombohedral angle &agr;R=55.30°±0.06° as well as the atom‐position parametersu(Al, Cr) =0.3520±0.0004 for Al (or Cr) andu(0) =0.556±0.003 for oxygen remain constant up to at least 86 kbar. These results are in very good agreement with earlier measurements up to 25 kbar by Schiferl and Jamieson as well as with the Birch and Murnaghan equations of state with the literature values of the isothermal compresssibilityK0and its pressure derivativeK′.