Reversing double-step strain deformations provide a severe test for constitutive equations. The rheology of reversible deformations is also of considerable importance for polymer processing simulations, specifically simulations of extrudate swell. We report double-step shear-strain experiments on two commercial (polydisperse) polymer melts, a (linear) PIE and a (long-chain branched) LDPE. Experimental data are compared to predictions of (1) a separable single integral constitutive equation with a damping function; (2) an irreversible temporary network model with a damping functional; and (3) the tube model of Doi and Edwards. None of the models is found to give a quantitative description of all of the data. However, irreversible network disentanglement can be explained quantitatively on the basis of the tube model if not only orientation but also stretch of molecular chains is taken into account. This is done by use of the molecular stress function introduced by Wagner and Schaeffer, and Doi＇s calculation for double-step shear-strain experiments is extended to include chain stretching. It is found that linear polymer melts like PIE show a limited amount of chain stretching and strong irreversible disentanglement, while enhanced chain stretching of long-chain branched LDPE melts prevents irreversible disentanglement at least up to shear strains of ten shear units.