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The purpose of this work was to study the role of cesium in sodium-based geopolymer and its thermal stability for nuclear waste management. A series of mixed sodium and cesium geopolymer samples (Na 1x Cs x ) 2 OÁAl 2 O 3 ÁSiO 2 Á12H 2 O (referred to as (Na 1x Cs x )-GP, where x = 0, 0.08, 0.15, 0.42, 1) have been prepared. All geopolymer samples were heated at 1100°C for 24 h. Pollucite (CsAlSi 2 O 6 ) and feldspathoid (CsAlSiO 4 ) were crystallized from Cs-GP. Nepheline (NaAlSiO 4 ) and a small amount of crystallized silica were obtained from Na-GP. The other geopolymers (Na 1x Cs x )-GP (x = 0.08, 0.15, 0.42) led to pollucite and nepheline main phases. Amorphous silica phase was observed in all the geopolymer samples with various amounts. Phase quantification and scanning electron microscope revealed that higher Cs concentrations in Na-GP tend to decrease the amorphous phase while improving pollucite and nepheline phase quantification. The amorphous geopolymers have also been studied by pair distribution function analysis. Tetrahedral chains formed by T-O bonding (with T = Si, Al) were shown to be more tighten around Cs + than around Na + . It led to shorter Cs-T bond than Na-T bond matching the higher solvation property of Na + . Furthermore, thermal study analysis pointed out the fact that geopolymer samples (Na 1x Cs x )-GP, can be considered as solid solutions.(2) Characterization X-ray diffraction pattern were recorded at RT in the 2h range 5-120°with a step size of 0.0167°using a X'Celerator Panalytical diffractometer (PANalytical, Limeil -Brévannes, E. Vance-contributing editor Manuscript No. 35172.