The tensile stress relaxation behavior subsequent to different imposed strains of poly(l-butene) was studied at room temperature. The plastic and elastic strains after a stress relaxation practically agree with those found for an immediate unloading. This indicates that the same decomposition of the total strain would also be found in an ideal quasi-stationary stretching experiment. Unloading curves as measured after a stress relaxation have a curved appearance indicative for a continuous change from forces of the crystal skeleton to forces of the entanglement network. Measured stress relaxation curves begin with an instantaneous drop, which can be associated with the cessation of a viscous flow, and then continue with a retarded relaxation process. The crystallite fragmentation process that sets in at the critical strain epsilon(H) = 0.7 leads to a rising value of the instantaneous drop. The retarded component can be related to the transition of the crystalline blocks from positions that they adopt during the stretching to the actual equilibrium positions. (C) 2004 Wiley Periodicals, Inc.