The electronic absorption spectra of Re2(piv)4Cl2 and Re2(piv)4Br2(piv = O2CCMe3) have been measured as a function of pressure in the crystalline solids and for the molecules dissolved in PMMA (poly(methyl methacrylate)). High-pressure infrared absorption spectra have been measured for the crystalline solids dispersed in mineral oil over the range 700-1800 cm-1. The major feature appearing in the electronic spectra is the growth in intensity of a band lying on the low-energy side of the δ→δ* excitation. This growth starts at the lowest pressures, while only above ~40 kbar is any decrease in intensity of the δ→δ * excitation observed. In the region above 25-30 kbar new peaks appear in the IR spectrum at 1134, 1280, and 1697 cm-1 for the bromide and at 1142, 1286, and 1697 cm-1 for the chloride. After consideration of alternative possibilities, the changes in the spectra are assigned to a molecular isomerization, in which the axial halide ligands move to radial positions and force two of the bidentate pivalate ligands to become monodentate.