Interfacial bonding force at the interface of polystyrene is computed by considering interaction of repeat units constituting molecules on the two sides. The interaction force between the repeat units is obtained from the 6–12 Lennard-Jones potential curves determined previously. The maximum value of the bonding force is of the same order of magnitude as the yield stress. Results indicate that regions in the immediate vicinity of flat regions where separation is within about 20A contribute to the overall strength of the bond. © 1969 Taylor & Francis Group, LLC. All rights reserved.