The structure of Cs2ZrSi3O9 synthesized using a sol-gel method was determined from the Rietveld refinement of experimental powder X-ray diffraction data. The refinement confirmed that this compound was Isostructural with wadeite: its structure was hexagonal (space group P6(3)/m), and its lattice parameters were a = 7.2303(2) Angstrom and c = 10.2682(4) Angstrom. The aqueous durability of Cs2ZrSi3O9 varied, depending on the solution conditions. In modified leach tests with buffered (pH 7) and unbuffered solutions, the 7-d cesium release rates were <1.2 x 10(-4) g (m(2).day)(-1), which indicated high aqueous durability. However, in unsaturated, unbuffered solutions with a pH of 9-10, the durability was much lower, with 7-d cesium release rates of 2.2 x 10(-3) g.(m(2).day)(-1). The ability of this material to retain cesium in aqueous environments can be explained by its condensed ring structure, in which the size of the channel openings is smaller than the diameter of a Cs+ ion. However, dissolution of the network silicate occurred at high pH, which resulted in the release of cesium.