The structure of a seven-ring model for the crystalline polyetherketone known as ＇PK99＇, has been determined by single crystal X-ray methods. The central benzene-1,3-dicarbonyl group in this oligomer adopts an ＇open＇ conformation, approximately in agreement with that proposed earlier for PK99 on the basis of X-ray powder and fibre diffraction data. The present oligomer structure land a polymer structure simulated by extrapolation from the oligomer) do not, however, require the benzene-1,3-dicarbonyl group to adopt the previously assumed coplanar conformation. A single crystal X-ray study of a cyclic ether-ketone oligomer, also based on the benzene-1,3-dicarbonyl unit, demonstrates the potential of this group to act as a site for sharply re-entrant chain-folding.