The defect structures for Gd- and La-doping on the A-site of BaTiO3 with the creation of Ti vacancies are reported. The rare-earth cations cluster together around the vacancy. The local relaxations caused by the defect cluster lead to distortion and tilting of nearby TiO6 octahedra and this is responsible for the lowering of the Curie temperature with increasing dopant concentration. Larger distortions to the octahedra are observed for La-doping and this is the proposed origin of the greater decrease in Tc as observed by experimental results associated with La- compared to Gd-doped samples.