The interdiffusion of two miscible polymers, polycarbonate (PC) and a copolyester (KO-DAR), was studied at temperatures from 200 to 230-degrees-C. The two polymers were coextruded as microlayer composites with up to 3713 alternating layers. The microlayer structure provided a large area of intimate contact between the two polymers with minimal mixing. Initially, two glass transition temperatures were observed by DSC that were intermediate between the glass transition temperatures of the pure components. Upon annealing, the glass transition temperatures shifted closer together, reflecting the extent to which interdiffusion had occurred. After no more than 2 h of annealing, a single glass transition temperature was observed. A model was formulated based on Fick’s law of diffusion that related the mutual diffusion coefficient, D, to the change in the glass transition temperatures. The model also incorporated an "equivalent" residence time to account for diffusion that occurred during the coextrusion process. It was not necessary to consider the concentration dependence of D to satisfactorily describe the data with this model. For the temperature range from 200 to 230-degrees-C, the value of D varied from 4.0 X 10(-16) to 1.6 X 10(-15) m2 /s. The activation energy of interdiffusion was determined to be 95 kJ/mol.