The mechanical properties of rare earth tritide films evolve as tritium decays into 3He, which forms bubbles that influence long-term film stability in applications such as neutron generators. Ultralow load nanoindentation, combined with finite-element modeling to separate the mechanical properties of the thin films from their substrates, has been used to follow the mechanical properties of model ErT2 films as they aged. The size of the growing 3He bubbles was followed with transmission electron microscopy, while ion beam analysis was used to monitor total T and 3He content. The observed behavior is divided into two regimes: a substantial increase in layer hardness but elasticity changed little over ∼18 months, followed by a decrease in elastic stiffness and a modest decease in hardness over the final 24 months. We show that the evolution of properties is explained by a combination of dislocation pinning by the bubbles, elastic softening as the bubbles occupy an increasing fraction of the material, and details of bubble growth modes.