AbstractA lap shear joint method was used to study strength development during welding of polystyrene surfaces‐ The surfaces previously had not been in contact and care was taken to insure rapid wetting of the interface. The shear stress at failure, τfwas measured at room temperature as a function of contact time,t, at constant welding temperatures up no 20°C above the glass transition temperature,Tg. The time dependence of welding could be well described by τfαt1/4. This result is in agreement with predictions of the reptation molecular dynamics model applied to inter‐diffusion at a symmetric amorphous polymer‐polymer interface. The activation energy for the thermally activated increase in strength development was determined asE= 96 kcal/mol atT= 113.5°C, which compares withE= 93,2 kcal/mole as predicted by the W‐L‐F theory using C1= 13.7, C2= 50 andTg= 100°C. The polystyrene samples had molecular weights,Mn= 143,000 andMw= 262.000. The time to achieve complete healing,t∞≈︁ 256 min at 118°C, was found to be of the same order of magnitude as the viscoelastic relaxation time and also with the time required for a polymer chain to diffuse a distance equal to its root mean sq