Plutonium and Pu-Ga alloys have been observed to have anomalous hydrogen solubility behavior, including a significant concentration dependence of hydrogen diffusivity in the dilute regime, a sharp drop off in the hydrogen solubility constant in the dilute regime, and a near complete absence of change in the Sieverts’ constant as the alloys are heated across phase transformation boundaries. We are investigating the possibility that a vacancy mechanism is responsible for this behavior. X-ray diffraction measurements show a 0.14% lattice contraction in Pu-2 at. % Ga alloys when they are charged with ~2 at. % hydrogen. The lattice re-expands when the hydrogen is removed. Density functional calculations show that increasing the number of hydrogen atoms associated with a vacant lattice site in Pu lowers the energy of the hydrogen-vacancy complex. These observations support the idea that vacancies are stabilized by hydrogen in the Pu lattice well beyond their thermal equilibrium concentration and could be responsible for the anomalous hydrogen response of Pu.