The crystal structure and morphology of the commercial ethylene-propylene-ketone terpolymer has been investigated by X-ray diffraction, DSC, and transmission electron microscopy. Crystallization from both the bulk and from dilute solution was considered. In both cases crystallinities were found to be <50%. Two crystal structures were observed, both based on those displayed by the perfectly alternating copolymer of ethylene and ketone units. The phase characteristic of the perfectly alternating copolymer under ambient conditions (the a structure) is seen as a minor component in the terpolymer. Rather, the prevalent phase in the terpolymer is the so-called beta phase, otherwise found in the imperfectly alternating ethylene ketone copolymer or in the perfectly alternating copolymer at elevated temperature.