Crystals of gold trifluoride are hexagonal, a= 5·149, c= 16·26 A, U= 373·3 A3, Dm= 6·72, Z= 6, Dc= 6·78, space-group P6122–D62. The structure was determined from a Patterson projection, and refined by three-dimensional least squares, the final R being 0·090 for 193 reflections.Gold trifluoride is a fluorine-bridged polymer, with gold atoms linked to square-planar arrangements of fluorine atoms. Each square-planar unit is linked to two adjacent units by cis fluorine bridges, giving an infinite hexagonal helix, with Au–F (bridging)= 2·04 A, Au–F (non-bridging)= 1·91 A, Au–F–Au = 116°. Weak cross-linking between chains, with Au⋯F = 2·69 A, completes a tetragonally-elongated octahedral environment around each gold atom.Gold trifluoride is the least densely packed of the noble-metal trifluorides. This and the geometry of the fluorine ligand co-ordination are consistent with the antibonding nature of one of the electron pairs of the low-spin d8 electron configuration of gold(III).