The diffusional mobility of disodium fluorescein has been measured in supercooled aqueous solutions of trehalose, a widely used cryoprotectant disaccharide. The results were analyzed on the basis of the classical continuum hydrodynamic theory (Stokes-Einstein relationship) and compared with results for the diffusion and electrical conductivity of other ionic and nonionic solutes in trehalose and sucrose aqueous solutions. Disodium fluorescein obeys the classical model over a restricted range of inverse reduced temperatures, T_g/T, scaled by the glass transition temperature. Decoupling in neutral solutes takes place at higher values of T_g/T, while in ionic solutes it occurs all over the range of T_g/T studied, as observed for the water mobility in supercooled sugar solutions.