Electron Spin Resonance (ESR) experiments in the filled Cei 1-xYbxFe4P12 (x ≲ 0.0023) skutterudite compounds of Th cubic symmetry (Im3) reveal the coexistence of two distinct Yb3+ sites with large difference in their site occupation, hyperfine interaction and temperature dependence of the ESR parameters. These results suggest a scenario where the oversized (Fe 2P3)4-cages of the Ce1-xYb xFe4P12 compounds are occupied with small number of on-center-Yb3+ ions and a distribution of highly occupied off-center-Yb3+ions, rattling at ∼1 GHz and above ∼20 GHz, respectively. © 2010 IOP Publishing Ltd.