Differential scanning calorimetry and high temperature x-ray diffraction were used to study the perfectly alternating copolymer of ethene and carbon monoxide (polyketone; POK-C2). It was found that oriented POK-C2 fibers show a crystalline phase transition at a temperature between 110-125 degrees C with a 10% change in crystalline density. At this temperature, the crystal structure reported recently (POK-alpha) is transformed to a crystal structure that was reported in the past for room temperature imperfectly alternating polyketone. The latter structure will be designated as POK-beta. The influence of chain defects on the crystal structure was studied by synthesizing terpolymers (POK-C2/C3), in which small amounts of propylene-CO units are incorporated into the polymer backbone. The resulting terpolymers differ from the copolymer by the presence of methyl groups randomly distributed along the polyketone backbone chain. Evidence is presented that indicates that the methyl groups are built into the crystal lattice as defects. With more than 5 mole-% propene the terpolymer fibers crystallize exclusively in the beta-modification. Below this level the alpha/beta ratio (at room temperature) increases with decreasing amounts of propene. Both as-synthesized and as-spun POK-C2 were found to consist of both POK-alpha and POK-beta; the alpha/beta ratio depends on the method of preparation. Because the drawn POK-C2 fibers studied here consist exclusively of POK-alpha, the process of spinning and drawing leads to the transformation of unoriented beta-rich material into oriented POK-alpha.