Concomitant polymorphism in the pseudo-peptide Me2N-pC6H4C(O)-Phe-OEt

► Monoclinic (1m) and orthorhombic (1o) polymorphs of pseudo-peptide 1 were studied. ► 1m and 1o reveal equivalent 2D structures with a different mode of layer stacking. ► Thermodynamic relations between two polymorphs were unambiguously established. ► Both polymorphs were prepared in a pure form from solution or by solvent-free methods. ► Kinetically stable 1m transfers to thermodynamically stable 1o by grinding or heating.