The salt, [F4S=NXe][AsF6], has been synthesized by the solid-state rearrangement of [F3S NXeF][AsF6] and by HF-catalyzed rearrangement of [F3S NXeF][AsF6] in anhydrous HF (aHF) and HF/BrF5 solvents. The F4S=NXe+ cation undergoes HF solvolysis to form F4S=NH2+, XeF2, and the recently reported F5SN(H)Xe+ cation. Both [F4S=NXe][AsF6] and [F4S=NH2][AsF6] have been characterized by Xe-129 and F-19 NMR spectroscopy in aHF and HF/BrF5 solvents and by single-crystal X-ray diffraction. The [F4S=NXe][AsF6] salt was also characterized by Raman spectroscopy. The Xe-N bond of F4S=NXe+ is among the shortest Xe-N bonds presently known (2.084(3) angstrom), and the cation interacts with the AsF6- anion by means of a Xe---F-As bridge in which the Xe---F distance (2.618(2) angstrom) is significantly less than the sum of the Xe and F van der Waals radii. Both F4S=NXe+ and F4S=NH2+ exhibit trigonal bipyramidal geometries about sulfur, with nitrogen in the equatorial plane and the nitrogen substituents coplanar with the axial fluorine ligands of sulfur. The F4S=NH2+ cation is isoelectronic with F4S=CH2 and, like F4S=CH2, has a high barrier to rotation about the S=N double bond and to pseudorotation of the trigonal bipyramidal F4S=N-moiety. The solution and solid-state rearrangements of F3S NXeF+ to F4S=NXe+ are proposed to result from attack at sulfur by fluoride ion arising from HF in solution and from the AsF6- anion in the solid state. Quantum-chemical calculations were employed to calculate the gas-phase geometries, charges, bond orders, valencies, and vibrational frequencies of F4S=NXe+ and F4S=NH2+. The F4S=NXe+ cation provides the first example of xenon bonded to an imido-nitrogen, and together with the F4S=NH2+ cation are presently the only cations known to contain the F4S=N-group. Both cations are intermediates in the HF solvolysis pathways of F3S NXeF+ which lead to F5SN(H)Xe+ and F5SNH3+, and significantly extend the chemistry of the F4S=N-group.