The aromatic poly(thioether-ketone) [ArSArCO](n) (Ar = 1,4-phenylene), PTEK, is known to exist in at least two different crystalline forms, one produced by orientation of the amorphous polymer and the other by crystallization from the melt. In the present work, X-ray powder diffraction studies interfaced to molecular modeling and diffraction simulation techniques have been used to determine the crystal and molecular structure of melt- or solution-crystallize PTEK. In this phase, the thioether-ketone polymer chains adopt a packing arrangement very different from that of their ether-ketone analogues. For solution-crystallized polymer, the unit cell is body-centered monoclinic (two chains per cell), space group I2, a = 7.58, b = 6.09, c = 10.25 Angstrom, beta = 93.7 degrees, density = 1.49 g cm(-3). The thermal parameters required for this model are however rather high, suggesting the possibility of a disordered structure tin space group I2/a,) whereby thioether and ketone linkages are distributed at random over the available sites. Melt crystallization results in a very slight expansion of the crystal lattice in the a-direction, relative to the solution-crystallize form, but the structure is otherwise unchanged.