Abstract The mechanism of strontium fixation on hydrated antimony pentoxyde ( Fd3 m space group) has been specified from thermodynamical interpretation of sorption isotherms and crystal structure. When the ionic fraction increases in the solid, two types of protons (protons in H 3O + groups and protons bonded to framework oxygen) are successively exchanged. In acid media (pH < 3) the uptake of Sr 2+ is limited to 0.31 mole/mole, and Sr 2+ is exclusively located in S(I) sites ( x = y = z ∼ 0.5). In more basic media (pH > 3), Sr 2+ is retained in S(I) as long as the concentration in the solid is less than 0.31 mole/mole. Beyond this value S(II) sites ( x = y = z = 0.300) are occupied up to a total capacity of 1 mole/mole. Thermodynamical data were calculated for each of these exchange steps.