AbstractA series of experimental data for crystallization and melting under pressure, obtained by using different measuring systems, are reconsidered on the basis of topoenergetic principles recently established. Two transformations statistically contribute at least in the global processes in high density polyethylene and trans‐1,4‐polyisoprene samples, namely: (i) the lamellar segregation as a result of molecular fractionation and/or defect concentration, and (ii) the lamellar thickening. These processes show a transition in their nature and/or amplitude for a value of approximately 200 MPa for the externally applied pressure during crystallization or melting. This transition is explained by morphological changes in the amorphous‐crystalline cou