The mechanism of tensile deformation in a semi-crystalline polymer like PE changes at four critical points which can be associated with (A) the onset of single slip-processes, (B) a turnover into a collective activity of slips, (C) the beginning of a disaggregation of the crystal blocks, followed by fibril formation, and (D) the onset of chain disentangling. Studies of the deformation behavior of series of LPEs with increasing molar mass show an abrupt rise of the critical strains epsilon(H)(C) and epsilon(H)(D) at a molar mass of M-t approximate to (1 5-2) X 10(5) g mol(-1). As indicated by dynamic mechanical tests the rise originates from a change in the mobility and compliance of the amorphous regions, which are melt-like for M-w < M-t, but reduced for M-w < M-t. As a consequence, both, crystal block disaggregation and chain disentangling, set in at lower strains. (C) 2003 Elsevier Science Ltd. All rights reserved.