When a coextruded multilayer sheet is pulled in a uniaxial extensional rheometer, the interfacial area per unit volume increases, amplifying interfacial effects. To investigate interfacial reaction, a pair of functional and miscible polyethylenes were co-extruded into multilayer sheets (256 layers). One polyethylene contained 8 wt% glycidyl methacrylate comonomer the other 4 wt% anhydride grafts. The extensional force to pull the molten multilayer sheet was measured via a rotating clamp extensional rheometer (RME, Rheometric Scientific). Owing to the reaction in the interfacial region before and during the extension, the measured extensional force increased dramatically. The interfacial stress was extracted and correlated to the extent of interfacial crosslinking. The rate of interfacial crosslinking was found to exceed the rate of area generation at extension rates < 0.01 s(-1). By annealing samples and then stretching them, the growth rate of the interlayer was determined, and a diffusion coefficient estimated.