We use polarization-resolved mid-infrared pump–probe spectroscopy to study the dynamics of the hydration shells of hydroxide ions (OH–). We excite the OH stretch vibrations of H2O molecules solvating the OH– ion and observe that this excitation decays with a relaxation time constant T1 of 200 fs. This relaxation is followed by a thermalization process that becomes slower with increasing concentration of OH–. The prethermalized state is observed to be anisotropic, showing that the energy of the excited OH stretch vibrations is dissipated within the hydration complex. The anisotropy of the prethermalized state decays both as a result of the reorientation of the OH– hydration complex and heat diffusion from the excited complexes to unexcited complexes. Modeling the anisotropy data at different concentrations allows for an accurate estimate of the number of water molecules in the hydration shell of OH–, the reorientation dynamics of the OH– hydration complex, and the molecular-scale heat diffusivity.