The possibility of using ferrocyanide sorbent based on hydrated titanium dioxide as a matrix for concentration and joint immobilization of caesium and strontium radionuclides is studied in the article. Chemical stability of sorbent samples saturated by radionuclides is determined via long-term leaching method and thermal behaviour of samples is studied. High chemical stability of sorption-active matrix with respect to leaching of caesium and strontium radionuclides by various types of water (distilled, tap, mineral and sea water) as well as thermal stability within the range of 20–100 °C are shown. Distilled water is the most aggressive leachant sorption-active matrix; for distilled water leaching rates ranged from (3.7 ± 0.2) × 10−10 to (8.2 ± 0.2) × 10−12 g cm−2 day−1 for caesium and from (1.8 ± 0.1) × 10−10 to (1.2 ± 0.1) × 10−12 g cm−2 day−1. Burial or storage of saturated sorption-active matrices dried at temperature of up to 100 °C is recommended.