The microhardness of low density polyethylene (LDPE) fractions and of high density polyethylene (HDPE) samples has been studied as a function of molecular weight in the range of about 2 x 10(4) up to 4 x 10(6). Details of the lamellar structure were determined by transmission electron microscopy (TEM). The observed decrease of hardness with increasing molecular weight is mainly due to the increase in thickness of the interlamellar layers (i.e. a decrease of crystallinity). After chemical treatment with chlorosulfonic acid and with OsO4 the samples show a drastic hardness increase. The kinetics of OsO4 treatment as revealed by microhardness has been examined. The hardness increase is explained in terms of the large reduction in molecular mobility of the amorphous, interlamellar layers. The strongest hardening effects arise in consequence of the preferred reaction of these defect layers with increasing molecular weight.