Density, viscosity, and diffusivity data have been obtained for equimolar ferri and ferro-cyanide (0.01–0.2 M) in 0.5 M aqueous sodium hydroxide solutions in the temperature range 10 to 90°C. For the determination of the diffusivity of the ferricyanide ion, limiting currents were measured by means of a rotating disc electrode, and the apparent diffusivities calculated from these measurements were corrected taking into account the migration term which became significant at high concentration. All data have been presented both graphically and as second order polynominals.