Poly(ether ketone ketone) (PEKK) with all-para linkages in phenylene groups is one member of the poly(aryl ether ketone) family. This polymer has been reported to exhibit two polymorphisms (forms I and II) based on wide-angle X-ray diffraction (WAXD) experiments. The form I crystal structure has been determined to have two-chain orthorhombic packing with a = 0.767 nm, b = 0.606, and c = 1.008 run. For form II, however, two different structures have been proposed : both are orthorhombic, but with different size and symmetry.1,2 Through our morphological observations, two different crystal forms have been isolated. Using electron diffraction (ED) experiments combined with WAXD, we have found that form II of PEKK should have a two-chain orthorhombic lattice. This unit cell dimension is consistent with the form II cell reported by Blundell et al.,1 namely, a = 0.417 nm, b = 1.134 nm, and c = 1.008 nm. Two maps of the formation of these forms with crystallization conditions are also discussed.