AbstractA large variety of nickel‐based catalysts has been investigated for the oxygen evolution reaction (OER) in alkaline media. However, their reported activity, as well as Tafel slope values, vary greatly. To understand this variation, we studied electrodeposited Ni₈₀Fe₂₀OOH catalysts with different loadings at varying rotation rates, hydroxide concentrations, with or without sonication. We show that, at low current density (<5 mA cm⁻²), the Tafel slope value is ≈30 mV dec⁻¹for Ni₈₀Fe₂₀OOH. At higher polarization, the Tafel slope continuously increases and is dependent on rotation rate, loading, hydroxide concentration and sonication. These Tafel slope values are convoluted by non‐kinetic effects, such as bubbles, potential‐dependent changes in ohmic resistance and (internal) OH⁻gradients. As best practise, we suggest that Tafel slopes should be plotted vs. current or potential. In such a plot, it can be appreciated if there is a kinetic Tafel slope or if the observed Tafel slope is influenced by non‐kinetic effects.