Mass transport is an important factor in the deposit quality of copper electrowinning. Presently, there is limited diffusivity data available at commercially relevant concentrations between 25 and 40 °C. Linear sweep voltammetry at a rotating disk electrode was used to measure effective diffusion coefficients of cupric ion for a wide range of copper concentrations (10-50 g/L), sulfuric acid concentrations (120-240 g/L), and temperatures (25-60 °C). The results were well correlated by the equation: D, m2/s = 2.977 x 10-10 -5.462 x 10-13 [Cu]-1.212 x 10-12 [H2SO4] + 1.688 x 10-11 x T, where [Cu] and [H2O4] are in g/L, and T is °C. Addition of 20 mg/L Cl- slightly increased effective diffusivity. Other common commercial organic smoothing agents were found to have no effect. The measured diffusivities were used to calculate the "maximum permissible current density" that can produce smooth dense cathodes as a function of copper concentration and temperature.