The thermal behavior, morphology of the melt-grown crystals, and polymorphism of poly(ketone sulfide) (PKS) were studied and compared with those of poly(ether ketone) (PEK), using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and wide-angle x-ray dif- fractometry (WAXD). The following results were obtained: (1) The melting temperature of PKS is somewhat lower than that of PEK, suggesting that the sulfide linkage (‒S‒) has a relatively shallower rotational conformational energy than that of the ether linkage (‒O‒). (2) The spherulites of PKS and PEK are composed of long, narrow lamellar crystals. It has been confirmed by electron diffraction (ED) studies that the longitudinal, transverse, and thickness directions of the long lamellar crystals correspond to theb, a, and c-axes, respectively. (3) The melt-grown and the melt- quenched and subsequently heat-treated PKS and PEK have a similar two- chain orthorhombic unit cell. The orthorhombic unit cell parameters of PKS area= 0.767 nm,b= 0.620 nm, andc= 1.044 nm. On the other hand, drawn or rolled PKS films have a two-chain monoclinic unit cell with parameters ofa= 0.836 nm,b= 0.606 nm,c= 1.044 nm, and γ = 68.5°. These results indicate that a crystal (orthorhombic)-crystal (mono- clinic) transition occurs during the deformation of PKS. The crystal structure of PEK does not change at all during deformation.