The morphology of unimodal and bimodal high-density polyethylene (HDPE) pipes during a hydrostatic pressure test was studied in detail using H-1 solid-state NMR. Characterizing the changes of the molecular network during such a test is of key importance for understanding the long-term properties of different HDPE pipe grades. The changes in amount, thickness, and molecular mobility of the crystalline phase, the interface, and the amorphous phase of the two pipe grades with the storage time have been quantified for the first time. The most sensitive microscopic parameter to storage is the molecular mobility of the amorphous phase, with the strongest changes shown by the unimodal HDPE. The density of the tie-molecules is not the main factor controlling the very different behavior of the two pipe grades, but rather it is the density of the entanglements. The NMR results offer unprecedented insights into the changes in the molecular network and support existing deformation models. (C) 2014 Elsevier Ltd. All rights reserved.