Diffusion coefficients are measured for PET-EG system at 265℃ and for PET-EA system for temperature range of 262-273℃. Experiments were carried out using a quartz spring sorption apparatus which was modified for high temperature measurements using high temperature silicon oil as a heat transfer fluid. We developed a simple mathematical model for the reaction combined diffusion because PET-EG system involves reactions with diffusional sorption. However, PET-EA system was a diffusion controlling process with little reaction. Experimental data for these two systems were compared with each other. Diffusion coefficients were obtained by considering the effect of sample depth in order to reduce the effect of reaction. From this method, diffusion coefficient in PET-EG system at 265℃ was $5.34\times10^{-5}cm./min$ and in PET-EA system at 262℃ was $4.7\times10^{-4}cm/min$. Temperature dependence on solubility in these two systems showed general phenomena and pressure dependence on solubility showed that Henry's law is valid. Apparatus used in this experiment also could be applied other polymer melt system of which the melting point was less than 290℃.
