An investigation into the reactivity of ArX (X = Br,Cl) found no evidence for successful intercalation of ZrCl between the chloride layers with NH(,3(g)) (up to 300(DEGREES)C), pyridine (190(DEGREES)), CO(,(g)) (170(DEGREES)), n-butyl lithium (25(DEGREES), 60(DEGREES)), Li/NH(,3) (-33 to -78(DEGREES)), K/NH(,3) (-33 to -78(DEGREES)), and Li (850(DEGREES)); and Zr plus LiCl (700(DEGREES)), CsCl (780(DEGREES)), and NaCl (930(DEGREES));Reactions of ZrX + n ZrO(,2) under isothermal conditions at 850-1000(DEGREES) for two weeks in welded tantalum containers produced an expanded ZrX structure (ZrX(O(,y))) and the metal phase Zr(O(,x))(X(,p)) (p (TURNEQ) 0). For ZrCl(O(,y)) saturation occurred at n = 0.27 (+OR-) 0.01 where the average cell dimensions are a = 3.4956(5) and c = 27.06(1) (ANGSTROM) (Guinier data) (a = 3.433(5), c = 26.693(3) (ANGSTROM) for ZrCl). For ZrBr(O(,y)), saturation occurred at n = 0.22 (+OR-) 0.02 where the average dimensions are a = 3.5584(4) and c = 28.430(7) (ANGSTROM) (a = 3.5064(2), c = 28.068(2) (ANGSTROM) for ZrBr);The a lattice dimensions of the metal phase from both halide systems were found to vary similarly to that of Zr(O(,x)) with increased oxide with the latter always 0.005 (ANGSTROM) larger. The a lattice dimension at saturation is in reasonable agreement to those cited above for ZrX(O(,y)). The c dimension of this phase saturated at a much lower oxide level with a noticeable increase of the bromide value (a = 3.239(2), c = 5.2189(9) (ANGSTROM)) over the chloride value (a = 3.2398(4), 5.2077(7) (ANGSTROM)), indicating possible halide incorporation although only a minute amount could be detected by electron microprobe;Three ZrCl(O(,y)) crystal structures were refined, y = 0.25(5), 0.29(4), and 0.43(2). One ZrBr(O(,y)) structure was partially refined. The oxide is incorporated into the tetrahedral-like interstices between the zirconium layers with no X-ray evidence of ordering. The ZrX structures with layers sequenced X-Zr-Zr-X do not change stacking order (ABCA for ZrCl and ACBA for ZrBr) or symmetry (R(')3M) with the addition of oxygen. ZrCl(O(,0.43(2))) represents the composition at saturation and has intralayer d(Zr-Zr = d(O-O) = d(Cl-Cl) = 3.4984(2) (ANGSTROM); and interlayer d(Zr-Zr) = 3.199(2) (ANGSTROM), d(Cl-Cl) = 3.648(5) (ANGSTROM), d(O-O) = 2.71(3) (ANGSTROM), d(Zr-Cl) = 2.673(2) (ANGSTROM), d(Zr(,apex)-O) = 2.15(2) (ANGSTROM), d(Zr(,base)-O) = 2.047(3) (ANGSTROM);The above information leads to the balanced equations at saturation ZrCl(s) + 0.27 ZrO(,2)(s) (--->) ZrCl(O(,0.43))(s) + 0.27 Zr(O(,0.42))(s) and ZrBr(s) + 0.22 ZrO(,2)(s) (--->) ZrBr(O(,0.35))(s) + 0.22 Zr(O(,0.42))(s); *DOE Report IS-T-1036. This work was performed under Contract No. W-7405-Eng-82 with the U.S. Department of Energy.