Defects can decrease the efficiency of scintillators by trapping electrons. Here, point defects in REAlO3 and RE3Al5O12 are predicted with pair potential simulations, where RE is yttrium or a trivalent rare earth cation. It was found that REAlO3 shows a preference for Al2O3-excess whereas RE3Al5O12 most readily exhibits RE2O3-excess. Also, lattice volume changes for the energetically favorable intrinsic mechanisms are relatively invariant as a function of RE cation size in RE3Al5O12, but not in REAlO3. However, in non-stoichiometric RE3Al5O12, the energetically preferred disorder mechanism results in an increasing lattice expansion with increasing RE radius whereas, in non-stoichiometric perovskites, a relatively small, radius independent, lattice contraction is predicted. These results illustrate that defect behavior in REAlO3 perovskites and RE3Al5O12 garnets is quite disimilar.