Crystals of F11PtXe are orthorhombic, a= 8·16, b= 16·81, c= 5·73 Å, Z= 4, space group Pmnb. The structure was determined from three-dimensional Patterson and electron-density maps, using Mo-Kα scintillation-counter data, and refined by least-squares methods; the final R-value is 0·130 for 549 observed reflexions. The structure can be formulated as [XeF5]+[PtF6]–. The [XeF5]+ ion has five fluorine atoms and a non-bonding electron pair in a distorted octahedral arrangement around Xe; in the [PtF6]– ion, Pt is co-ordinated octahedrally to six fluorine atoms. Four fluorine atoms from different [PtF6]– ions form bridges to the xenon atom, with Xe⋯F distances of 2·52, 2·65, and 2·95 Å(2), the directions of approach of the fluorines suggesting steric activity of the xenon non-bonding electron-pair.