In the "large aggregates" region of the phase diagram of the system C(12)E5/D2O the mutual diffusion coefficient of mixtures with critical and noncritical composition is determined by dynamic light scattering (compositions : 0.23 x 10(-2) less than or equal to y less than or equal to 10.01 x 10 (2); y, = 1.08 x 10(-2); y mass fraction of Cr(12)E(5)). In all experiments the diffusion coefficient decreases with increasing temperatures, approaching the liquid/liquid coexistence curve at fixed values of y. This is a system independent property observed in "simple" as well as in "complex" binary mixtures with a miscibility gap. The change of size and shape of C(12)E(5) micelles with composition and temperature in aqueous solutions reported for the "large aggregates" region appears to be the result of concentration fluctuations existing in a wide temperature and composition region below the binodal curve. Approaching T-p (T-c) the diffusive properly of the mixtures is dominated by the dynamics of concentration fluctuations.