Cadmium doping has recently emerged as an efficacious strategy for defect suppression and band gap tuning in hybrid as well as all-inorganic halide perovskites. However, the cadmium speciation in these materials is unknown. Here, we use high-field cadmium-113 NMR spectroscopy in conjunction with chemical shift calculations by fully relativistic density functional theory to establish the phase composition of cadmium-doped lead halide perovskites. We find that cadmium does not incorporate into the 3D perovskite lattice of MA- and FA-based lead halide perovskites (MAPbI(3) and the gold-standard triple cation mixedhalide composition). Instead, it forms separate, cadmium-rich nonperovskite phases for as little as 1 mol % Cd2+ doping. Conversely, we find that cadmium can incorporate into the 3D perovskite lattice of CsPbBr 3 via homovalent Pb2+ substitution up to around 10 mol %. Our results thus reveal the atomic-level mechanism of this recently developed defect passivation strategy.