Two alternating current (cc) methods arc presented for simultaneously determining the diffusivity and bulk concentration of a diffusing species in an electrolyte. The first technique is called the phase angle method, which consists of applying a sinusoidal ac in the frequency range of O.l-OSHz to a rotating disk electrode (rde) and measuring the phase lag of the resulting concentration overpotential changes at a given rotational speed. The diffusivity of the diffusing species may be calculated from the measured quantity without the need of its bulk concentration and information on the number of electron transfer on the electrode reaction. The second technique is called the limiting ac current density method, which consists of measuring the limiting ac current density on the rde in the ac frequency range of 20-3OOHz and plotting the ratio of limiting (IC current density to the steady-state dc limiting current density against the square-root of the ratio of ac frequency to angular speed of the rde. A straight line passing through the origin is obtained, and the diffusivity of the dithtsing ion may be calculated from the slope of the straight line using the kinematic viscosity of the electrolyte. Once the diffusivity is known, the bulk concentration of the diffusing species is obtained from the de limiting current on the rde using the Levich equation. The feasibility of the above UC methods was examined by measuring the difhtsivities and concentrations of ferricyanide and t&iodide ions in various supporting electrolytes in the temperature range of 20-50Β°C. The results were compared to the concentration and diffusivity values independently obtained by an iodometric titration and conventional dc limiting current technique on a rde. A reasonably good agreement was obtained between the results of ac methods and those of the chemical analysis and rde techniques. The root-mean-square deviation between the two sets of data was within f 15% for the diffusivity and f 13% for the concentration measurements.