A rheological method to assess the diffusion behavior of liquids in molten polymers above their boiling point is presented in this article. The evaporation of the liquid and the evolution of the blends complex modulus are followed by rheological measurement. Identification of the Fick's diffusion coefficient and the rate of evaporation of butylmethacrylate in low density polyethylene is done. The values of the diffusion coefficient are in agreement with those obtained by classical methods. The method developed here appears as complementary to the existent ones. It allows the measurement of diffusion coefficient of liquid/polymer system at high temperatures even for volatiles liquids. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 759-763, 2010