Mesoporous silica gel containing embedded phosphotungstic acid (PTA) was synthesized by sol-gel co-condensation of tetraethoxysilane with PTA in acidic media. The obtained material had high BET surface area and pore volume. A characteristic band of the Keggin structure of PTA was present in its FT-IR spectrum while its XRD patterns were absent. This proved the embedding of PTA on a sub-molecular level. The material demonstrated high adsorption capacity of Cs. Unexpectedly, porosity of the adsorbent increased after substitution of most protons by cesium cations. Cation exchange also favored agglomeration of the material particles. Kinetic studies showed that the adsorption data correlates strongly with the pseudo-second order model. The adsorbent had two types of adsorption sites: heteropolyacid anions and silanol groups. However, adsorption on silanol groups was very sensitive to the temperature. At the increased temperature, the nature of adsorption fit the Langmuir model extremely well. The obtained results can be used in the development of an effective adsorbent for clean-up of water contaminated by radioactive cesium-137.