Tracer diffusion coefficients of Co 2+, Ni 2+, Cu 2+ and H + ions in 0.5M aqueous Na 2SO 4 solutions at 25°C have been measured by using Stokes magnetically-stirred porous diaphragm cells. Changes in the concentrations of the diffusing ions were determined by potentiometric titration with EDTA (for Co 2+, Ni 2+ and Cu 2+) or NaOH(for H +. Linear extrapolation of the measured diffusion coefficients to zero concentration gave the tracer diffusion coefficient values 0.56 (±0.01) × 10 −9, 0.54 (±0.02) × 10 −9, 0.55 (±0.01) × 10 −9 and 2.67 (±0.01) × 10 −9 m 2 s −1 for the respective ions. The diaphragm cell method is simple and inexpensive, and the accuracy of the results is 1–2 orders of magnitude better than the accuracy of diffusion data obtained by the popular rotating disc electrode method.