La2‐xRExCuO4 (RE = Nd‐Y) solid solution systems have been investigated to determine the factors stabilizing the T (La2CuO4), T’ (Nd2CuO4) and hybrid T*‐type structures. A simple ionic model with a perovskite‐like tolerance factor is found to accurately define the existence field of each. Metastable T* phases are observed for the larger RE cations Nd, Gd, Eu. The structure is quite stable for RE = Dy, Tb, but does not occur at all for the smallest rare earths. Oxygen activity plays a role in T’ and T* phase formation.