Sodalites are one of the potential candidate forms for the radioactive ¹²⁹I immobilization. In this work, the synthesis, structure and thermal stability of iodine-contained sodalites Na₈(AlSiO₄)₆Cl_2-xI_x (x = 0–2) were studied. The high temperature solid phase method combined with high pressure tableting and NaOH solution scrubbing was proposed to synthesize iodine-contained sodalites with single phase. The X-ray diffraction Rietveld analysis shows that, the crystal structure of Na₈(AlSiO₄)₆Cl_2-xI_x varies as a function of the iodine content, x. The thermal stability of iodine-contained sodalites was studied by thermogravimetric analysis and high temperature annealing experiments. The iodine-contained sodalites exhibit good stability below 750 °C. A phase decomposition from the sodalite phase into the nepheline phase was observed for each composition at higher temperatures. The quantitative analysis of phase decomposition of Na₈(AlSiO₄)₆Cl_2-xI_x annealed at 900 °C for 3 h further confirms the negative effects of iodine-doping on the thermal stability of samples.