The aromatic poly(thioether-ketone) [ArSArCO](n)(Ar = 1,4-phenylene) is known to exist in two different crystalline forms, one produced by orientation of the amorphous polymer and the ether by heat treatment or crystallisation from the melt. In the present work, single crystal X-ray data for the oligomer [ArCOArSArCOAr] rc:veal that bond angles at the thioether and carbonyl linkages are substantially different, at 108 degrees and 121 degrees respectively, and that the linear chain-conformation required for polymer crystallisation is achieved by a series of bond-angle distortions elsewhere in the molecule-in particular by pyramidalisation of the carbon atoms adjacent to the thioether bridge. Crystal and molecular simulation of orientation-crystallised poly(thioether-ketone) shows that the unit cell is analogous to the "Form II" cell known for poly(etherketone)s. The polymer crystal structure thus has orthorhombic symmetry (two chains per cell), space group Pb2n, ct = 4.12, b = 11.30, c = 10.44 Angstrom, and density 1.45 g cm(-3). The previously reported monoclinic unit cell is shown to be based on a modification of the present orthorhombic lattice.