Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Electron-spin-resonance (ESR) measurements performed on the filled skutterudite system Ce1-xYbxFe4P12 (x less than or similar to 0.003) unequivocally reveal the coexistence of two Yb3+ resonances associated with sites of considerably different occupations and temperature behaviors. Detailed analysis of the ESR data suggests a scenario where the fraction of oversized (Fe2P3)(4) cages that host Yb ions are filled with a low occupation of on-center Yb3+ sites and a highly occupied T-dependent distribution of off-center Yb3+ sites. Analysis of the Yb-171(3+)(I=1/2) isotope hyperfine splittings reveal that these two sites are associated with a low (similar to 1 GHz) and a high (greater than or similar to 15 GHz) rattling frequency, respectively. Our findings introduce Yb3+ in T-h symmetry systems and use the Yb3+ ESR as a sensitive microscopic probe to investigate the Yb3+ ions dynamics.805Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)DOE [DE-FG02-98ER45707]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)DOE [DE-FG02-98ER45707]