Two packing polymorphs of 5-hydroxy-1-(4-methylbenzyl)-3-chloro-4-(4-chlorophenyl)sulfanyl-1,5-dihydro-2 H -pyrrol-2-one are studied by single-crystal and powder X-ray diffraction, IR spectroscopy, and differential scanning calorimetry. Classical hydrogen bonds and CL…O interactions have the fundamental importance for the formation of the supramolecular motif that is identical in both modifications. The difference relates to the packing of 2D layers which is “parallel” in the triclinic modification and of the “herringbone” type in the monoclinic modification. It is shown that close similarity between structural organizations of crystal lattices, which is typical of packing polymorphs, can be accompanied by significant differences between their energies as well as by a high potential barrier upon the transition from the metastable polymorph to its stable form.