Diffusion coefficients of several methyl esters of linear higher fatty acid (C-10 approximately C18) into polypropylene (PP) were determined, over the temperature range of 50 to 110-degrees-C, using a mass uptake technique. The relations between an amount of mass uptake and t1/2 were linear within 10 min, indicating Fickian diffusion. The diffusion coefficients were in the range of 4.6 X 10(-8) to 6.2 X 10(-12) cm2 sec-1, increased with temperature, and decreased with increasing n-alkyl chain length of fatty acid. The plots of In D vs. the molecular weight of methyl esters were approximately linear. The temperature dependence of diffusion followed an Arrhenius relationship, and the activation energies were in the range of 133 approximately 147 KJ mol-1 and increased with an increase in n-alkyl chain length of corresponding fatty acid. The diffusion coefficients were discussed with the interaction between methyl esters of higher fatty acid and PP. The diffusion coefficients of methyl esters, at the simultaneous diffusion process, were always higher than that derived from the single diffusion process, except for methyl decanoate with lower molecular weight. The difference in diffusion coefficients in single and simultaneous diffusion was discussed thermodynamically.