Amorphous bulk samples of high-molecular-weight (HMW) polystyrene (PS) with a weight-average molecular weight M-w of 102.5 kg/mol and a number-average molecular weight M. of 97 kg/mol and of ultra-high-molecular-weight PS (UHMWPS) with M-w=1110.5 kg/mol and M-n = 965.6 kg/mol were brought into contact to themselves below the glass transition temperature T-g of the bulk Tg-bulk, in a lap-shear joint geometry, at a constant healing temperature T-h for a healing time t(h) of 10 min to 24 h. The lap-shear strength sigma of the symmetric HMWPS-HMWPS and UHMWPS-URMWPS interfaces has been measured at the ambient temperature. Larger values of sigma have always been measured for the HMWPS-HMWPS interface after healing at the same conditions. However, the difference between the values of a, for the HMWPS-HMWPS and UHMWPS-UHMWPS interfaces observed at T-h = Tg-bulk - 33 degreesC and T-h = Tg-bulk - 23 degreesC became negligible at T-h = Tg-bulk - 13 degreesC at long times. The kinetics of strength evolution at the two interfaces involved at T-h = Tg-bulk - 33 degreesC and T-h = Tg-bulk - 23 degreesC was found to follow the form sigma proportional to (t(h))(1/4)(M-n)(-1/4), in accordance with the minor chain reptation model, suggesting that the build-up of the interface structure develops via the diffusion of chain segments above T-g of the interface layer Tg-interface. (C) 2004 Elsevier Ltd. All rights reserved.