Investigations of the reaction of CsI with excess ZrO(,2) in various containers at 400-850(DEGREES)C have successfully demonstrated the formation of ZrI(,4) and Cs(,2)ZrO(,3) in low yields (<35%) based upon the analytical evidence from X-ray fluorescence and atomic absorption techniques;To further investigate the chemistry, macroscopic amounts of the cesium zirconates have been synthesized from reactions of Cs-Zr-O(,2) in silver crucibles or of Cs-CsO(,2)-ZrO(,2) and CsO(,2)-Zr in welded silver tubing at 600-750(DEGREES)C for one to two weeks. These reactions have produced four new compounds: Cs(,2)ZrO(,3) and Cs(,4)ZrO(,4) each in 95-100% yield, and two evidently ZrO(,2)-richer cesium zirconates of unknown composition and structure in 40-50% yield;The crystal structures of the first two phases have been solved by single crystal methods. The Cs(,2)ZrO(,3) is structurally related to K(,2)ZrO(,3) or Cs(,2)PrO(,3) and crystallizes in the orthorhombic space group Cmcm with a = 11.256(6), b = 7.738(5) and c = 5.967(7) (ANGSTROM), Z = 4 (R = 5.2%). Its structure consists of infinite chains along the c-axis formed by square pyramidal Zr(O)O(,4/2) units sharing opposite basal edges and represents the second zirconate reported with five-coordinate zirconium. The compound Cs(,4)ZrO(,4) crystallizes in the monoclinic space group P2(,1)/c with a = 7.163(1), b = 19.919(2), c = 7.158(1) (ANGSTROM) and (beta) = 103.05(8)(DEGREES), Z = 4 (R = 3.2%). The structure contains isolated and slightly distorted tetrahedral ZrO(,4) units. The third zirconate crystallizes in a triclinic cell with a = 10.254(1), b = 10.416(2), c = 10.104(1) (ANGSTROM), (alpha) = 106.45(1)(DEGREES), (beta) = 98.51(1)(DEGREES), and (gamma) = 100.17(1)(DEGREES). Volume relationships suggest a composition near 3Cs(,2)O(.)7ZrO(,2). The fourth zirconate evidently crystallizes with a monoclinic cell, a = 9.926(5), b = 15.359(8), c = 20.642(8) (ANGSTROM), (beta) = 103.28(3)(DEGREES);The thermal stabilities of Cs(,2)ZrO(,3) and Cs(,4)ZrO(,4) have been investigated both in vacuo and in sealed silver containers. The compounds Cs(,2)ZrO(,3) and Cs(,4)ZrO(,4) decompose into Cs(,2)O and ZrO(,2) or Cs(,2)ZrO(,3) at near 915(DEGREES)C and 260-280(DEGREES)C under high vacuum, whereas in sealed silver containers they are stable to above 900(DEGREES)C and to 730 (+OR-) 20(DEGREES)C, respectively. The relevance of the formation and properties;of these zirconates to the distribution of fission products is also discussed; *DOE Report IS-T-1253. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.