The strong O-H⋯N and O-H⋯O hydrogen bonds involving two types of crystallographically different hexacyanoferrate(III) anions and water molecules in [GdIII(18-crown-6)(H2O)3][Fe III(CN)6]·2H2O result in 3-D assembly consisting of the novel example of interlocked infinite double helices. © The Royal Society of Chemistry 2005.