Doping structures of Ce3+ into the refractory alpha-sialon crystal lattice have been examined via an atom-resolved Cs-corrected scanning transmission electron microscope. The location and coordination of the rare-earth ions are well-defined through direct observation in conjunction with structural modeling and image simulation. The stability and solubility of Ce3+ ions could be remarkably enhanced via congregation into the planar defects formed by a 1/3 < 210 >-type lattice displacement along with an inversion operation. The formation of cylindrical chambers near the defects is believed to provide effective structural relaxation upon doping of large rare-earth ions into the interstices in their neighborhoods. The as-revealed structural information could be useful for understanding the luminescence properties of the promising rare-earth doped sialon materials.