The hydrothermally prepared hexagonal tungsten bronze (HTB) phase displays promising distribution coefficients (KD) for both Cs+ (2 - 100 ppm) and Sr2+ (0.5 - 60 ppm) in acidic (IM HNO3) radioactive waste simulants. The development of an inorganic ion-exchanger that displays such selectivity has previously eluded researchers in this field. The selectivity for Cs- and Sr2+ can be modulated by isomorphous substitution of molybdenum into the tungstate framework, and is optimum for material of nominal composition, Na0.2Mo 0.03W0.97O3·zH2O (Mo-HTB). Both the parent HTB and Mo-HTB phases display fast ion-exchange kinetics for Cs+ and Sr2- and cation exchange capacities ca. 50% that of the theoretical capacities of 0.9 and 0.45 mmol.g-1, respectively. The Mo-HTB adsorbent has a modest tolerance to alkali metal ions such as Na- and K- in acidic solutions with total Cs+ and Sr2+ uptake dropping by 66% as the concentration of Na + increases from 9 mmol.L-1 to 1200 mmol L-1.