A simple method to investigate polymer crystallization during fast cooling, based on in sitar temperature acquisition and ex-situ structural characterization, is proposed. The approach enables one to obtain the continuous cooling curve (CCC) diagrams, widely used in metallurgy but seldom adopted for semicrystalline polymers. This method is here exploited to gain new insights on polymorphic behavior of quenched polypropylene and its copolymers with ethylene. Experimental CCC diagrams, covering a wide range of crystallization temperatures in the domains of monoclinic structure and mesophase, are obtained for the first time. The role of counits in affecting the development of the mesophase upon fast cooling is assessed: the critical cooling rate above which a predominant fraction of mesomorphic form is generated significantly decreases with increasing comonomer concentration. This is due to the remarkable hindrance of ethylene counits on the crystallization kinetics of the a-form, which indirectly favors the development of the less affected mesophase. We expect that this concept can be extended to any kind of defects that disturbs the structuring of the monoclinic phase.