The polymorphic structure and the thermal transitions of several iPP-1-hexene copolymers have been analyzed by performing DSC and variable-temperature X-ray diffraction experiments. Special attention has been paid to the range of "intermediate" comonomer contents, between around 8 and 12 mol %, where the new trigonal form of iPP starts to be observed. It was found that the mesomorphic modification is also easily obtained in that comonomer range, so that the possibility of such mesomorphic phase acting as a transient structure of the trigonal form was entertained. However, the results have clearly shown that the mesomorphic modification is not acting as such transient structure, and rather it is a competitor (as well as the alpha crystals) for the formation of the trigonal phase.