The lamellar crystal morphology of thin films of poly(aryl ether ketone ketone) having a meta-linkage (PEKK(I)) is observed under transmission electron microscopy (TEM), utilizing samples with different thermal histories and preparation procedures. After isothermal crystallization from the melt, a double endothermic behaviour can be found through differential scanning calorimetry (d.s.c.) experiments. Partial melting of PEKK(I) can be obtained by heating the samples to a temperature between these two endotherms. By comparing the TEM morphological observations of the samples before and after partial melting, it can be shown that edge-on lamellar crystals, having different thermodynamic stabilities, form during isothermal crystallization. After partial melting, only the type of lamellar crystal exhibiting the higher thermodynamic stability remains. Small angle X-ray scattering (SAXS) results exhibit a change in the long period of the lamellar crystals before and after the partial melting process. For the solution-induced crystallization of PEKK(I), only a single endotherm exists in the d.s.c. observations. After annealing at high temperatures, however, a double endothermic behaviour appears. This also results in a decrease in the long period observed by SAXS. The double population of the lamellar crystals is difficult to observe in TEM, since the lamellar crystals in these thin film samples possess a random, rather than an edge-on, orientation. Detailed TEM imaging for observing these lamellar crystals is discussed.