Melting enthalpies of several ethene-propene samples, obtained by different single-center catalytic systems and having widely different constitutions and compositions in the range of industrial interest, are compared. For the first time, a clear evidence of influence of the regio- and stereoregularity of propene insertion on crystallization behavior of ethene/propene copolymers is provided. This influence is in qualitative agreement with the results of simplified energy calculations relative to models of pseudo-hexagonal packing of ethene-propene copolymers.