The crystal phase transitions occurring in oligo(aryl ether ketone) samples were studied, in real time, by means of X-ray scattering using synchrotron radiation. One sample is a typical molecular compound with 7 aryl units (m7) while the other two samples, containing 11 aryl units (pll and mil), exhibit the presence of a polymeric component. In relation to the polymorphic transition previously reported for aromatic polyketones, the phase transition study of the sample pll clarifies the role of chain length in the thermal behavior of aromatic polyketones. The structural variations, due to crystal phase transitions, as revealed by X-ray diffraction during heating of the samples are not accompanied by the parallel changes in the DSC thermograms. The two endothermic peaks observed for the samples pll and mil are associated with the melting of crystals with short and polymeric chains. Although the amount of polymer component reaches about 75% in the case of pll (30% for mil), it does not exhibit the double melting phenomenon commonly observed for aromatic polyketones. A structural mechanism is suggested to explain the absence of such a phenomenon for the pll and mil samples.