The positions of Ar atoms encapsulated in the cavities of fully dehydrated Cs3Na8H-A (Cs-3-A) have been determined. Cs-3-A was exposed to 660 arm of argon gas at 400 degrees C, followed by cooling at pressure to encapsulate Ar atoms; a second crystal was treated similarly at 1000 atm. The resulting crystal structures, of Cs-3-A(5Ar) (a = 12.253(2) Angstrom, R(1) = 0.055, R(2) = 0.056) and Cs-3-A(6Ar) (a = 12.253(2) Angstrom, R(1) = 0.064, R(2) = 0.065), have been determined by single-crystal X-ray diffraction techniques in the cubic space group Pm (3) over bar m at 21(1) degrees C and 1 atm. The five and six hr atoms per unit cell, respectively, are distributed over three crystallographically distinct positions : one Ar atom at Ar(1) lies opposite a six-ring in the sodalite unit, two Ar atoms at Ar(3) occupy a similar position in the large cavity, and two or three Ar atoms in Cs-3-A(5Ar) and Cs-3-A(6Ar), respectively, are at Ar(2) opposite four-rings in the large cavity. Relatively strong interactions of Ar(1) and Ar(3) atoms with six-ring Na+ ions are observed : Na-Ar(1) = 3.26 and Na-Ar(3) = 3.00 Angstrom. The four Ar atoms in the large cavity of Cs-3-A(5Ar) form a rhombus with Ar(2) = Ar(3) = 4.75(8) Angstrom and Ar(2)-Ar(3)-Ar(2) = 88(1)degrees. The five Ar atoms in the large cavity of Cs-3-A(6Ar) have a trigonal bipyramidal arrangement with three Ar(2) atoms at equatorial positions and two Ar(3) atoms at axial positions; Ar(2)-Ar(3) = 4.63(9) Angstrom. Both of the above arrangements are stabilized by alternating dipoles induced on Ar(2) and Ar(3) by four-ring oxygens and six-ring Na+ ions, respectively.