The mutual diffusion coefficient of a pair of polyethylenes using coextruded multilayers has been measured. Two polyethylenes with different molecular weight were coextruded in 32 alternating layers in about 45 s of contact time. When the multilayer sample was remelted in a parallel plate rheometer, its apparent viscosity increased due to interdiffusion. By following the kinetics of the apparent viscosity increase, the mutual diffusion coefficient was measured. The theory of Kramer et al. (Polymer. 1984; 25:473-480) was used to describe the mutual diffusion coefficient as a function of the chain mobility parameter, molecular weight, and concentration. The finite element method was used to solve the nonlinear interdiffusion problem to give a concentration profile in these multilayers. By fitting the apparent viscosity of the sample vs. time, the chain mobility parameter was found to be 4.5 x 10(11) m/N s, and the range of the mutual diffusion coefficient for these two PE's was 10(-12)-10(-14) m(2)/s at 200 degrees C (depending on concentration). This is an easy and rapid method to measure the mutual diffusion coefficient for miscible polymers with different melt viscosities. (c) 2007 American Institute of Chemical Engineers.