Alternative Er doping into anatase (A–) or rutile (R-)TiO2 can be achieved by O2 pressure control during Er-doped TiO2 deposition using laser ablation. An x-ray absorption fine structure analysis of the atomic coordination around Er revealed the dynamics of Er–O clustering in A– and R-TiO2. The Er local structure in A-TiO2 can be determined by a Ti–O arrangement of the host crystal surrounding Er, while that in R-TiO2 can be decided by the Er–O chemical properties rather than the Ti–O arrangement. Moreover, a substitution of Er for Ti induced O deficit in A-TiO2 and induced Ti removal in R-TiO2. These differences can be explained by the ErO6 stabilization mechanism based on the spontaneous symmetry reduction (SSR) theory. The Ti–O arrangement in A-TiO2 prevents the SSR, resulting in ErO6 instability. However, ErO6 is stabilized in the host crystal because the SSR is smoothly induced in R-TiO2. © 2003 American Institute of Physics.