AV-17(Cr) material was prepared by modifying a commercial polymer AV-17 (initially bearing R4N⁺ functional groups) with Cr(III) compounds was investigated. Comparative isotherms characterizing the CrO₄^2- and Cr₂O₇^2- ion sorption from solution with pH values of 3 and 8, respectively, at 19°C and 60°C, were obtained using the raw AV-17(Cl) and the modified AV-17(Cr) material. The sorption process on AV-17(Cr) takes place through the coordination of Cr(VI) ions with Cr(III) ions, while on AV-17(Cl) it is due to an ion exchange mechanism. The sorption isotherms obtained in pH 8 solutions fit perfectly the Langmuir model. The thermodynamic functions ΔG, ΔH, and ΔS characteristic to the adsorption of CrO₄^2- ions from K₂CrO₄ solutions with pH = 8 were calculated. The active sites belonging to the AV-17(Cr) material were identified as jarosite type compounds of Cr(III). SEM images show that the Cr(III) compounds are present in the form of ultrafine particles located on the surface and within the bulk phase of the polymer granules. Comparative thermal degradation process of both the AV-17(Cr) and the AV-17(Cl) material was conducted in air and in an inert atmosphere (He). The thermal degradation mechanism of sorbent AV-17(Cr) and exchanger AV-17(Cl) is proposed.