The phase 11 crystal structure of perdeuterated cyclohexene oxide has been solved from neutron powder diffraction data at 5 K utilising a simulated annealing method. The structure is triclinic, space group P1, with a unit cell volume of 268 angstrom(3). The refined molecular conformation is shown to be in close agreement with recent ab initio calculations. The p-T phase diagram determined using DTA and details of the phase I structure are also reported. (c) 2006 Elsevier B.V. All rights reserved.