Cyclic diamides with various CH2 segment lengths were synthesized. Cyclo-(octamethylene sebacamide) I, x = 8, was investigated as a Model system by means of calorimetry, X-ray diffraction and Raman spectroscopy with special emphasis on its phase behaviour. In the low temperature phase the molecules exist as collapsed rings where the amide groups are located in the folds and the straight stems consist of nondisordered CH2 segments. During the solid phase transition at about 84-degrees-C a strong increase of conformational disorder (4 additional gauche defects per molecule) occurs which does not change significantly during subsequent melting. It is shown that the conformation of the molecules in the high temperature phase is similar to that in the melt (liquid-like). In spite of conformational disorder the linewidth of the X-ray reflection peaks is smaller in the high temperature phase compared to that in the solution crystallized phase.