The diffusion process in dPS (M(w) = 1.06 x 10(6) g mol-1)/hPS (M(w) = 2.89 x 10(6) g mol-1) mixtures at constant temperature below the critical temperature slows down approaching the coexistence curve from the homogeneous single fluid phase region of the phase diagram. This effect can be understood in terms of the composition dependence of the thermodynamic factor partial derivativemu(dPS)/partial derivativePHI(dPS)T,P in combination with the Flory-Huggins theory of polymer melts. The theoretically expected decrease of the mutual diffusion coefficient with increasing temperature along the coexistence curve approaching the critical temperature is not observed in the temperature range studied (5 K < (T(c) - T) < 22 K). The temperature dependence of the diffusion coefficient D is dominated by the temperature dependence of the phenomenological transport coefficient L. The apparent activation energy of the diffusion coefficient of the polymer melts studied is larger by a factor of about 10 than that of binary mixtures of low molar mass components.