An attempt has been made to establish the role of diffusion across the polymer interface as a contributing factor to lap-shear bond strength. The assemblies studied were polypropylene/linear low-density polyethylene (PP/LLDPE); polystyrene/polyvinyl chloride (PS/PVC); polystyrene/polymethyl methacrylate (PS/PMMA); and polyvinyl chloride/polyvinylidene fluoride (PVC/PVDF). Initial bond strength measurements were followed by measurements on joints that had been annealed for up to 72 h at temperatures in the range of 60-160degreesC. Following induction times of tens of minutes where bond characteristics remained constant, substantial increments in bond strength were observed for PP/LLDPE and for PS/PVC but not for PS/PMMA or PVC/PVDF Results point to a significant contribution to bond strength arising from diffusion when dispersion forces and favorable acid/base interactions act at the interface. The times required to establish the apparent diffusion effects far exceed normal bonding times and may account for the failure of diffusion to be recognized as a significant mechanism in the formation and maintenance of adhesive bonds.