Large amounts of radioactive waste effluents are produced by the nuclear industry each year and more efficient methods are required for their treatment to decrease the final disposal cost and environmental discharge. Amongst the various methods, adsorption stands out as a relatively clean and energy-efficient separation method. Hexagonal tungsten oxide (hex-WO₃) has potential application to deal with acidic radioactive waste. To extend the adsorption capacity of hex-WO₃, an extensive series of Nb-doped WO₃ were synthesized using a hydrothermal method. Incorporation of Nb into the WO₃ framework leads to expansion of the layer spacing and is accompanied by the exclusion of guest water molecules. When compared to bare hex-WO₃, the adsorption capacity of Sr²⁺ on Nb-doped WO₃ is markedly improved and the Cs⁺ adsorption capacity not obstructed.