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A rigorous study of electrodeposited pure and mixed Ni/Fe oxides was performed in three different sodium hydroxide electrolytes with various Fe impurity concentrations (<1 ppb, 5 ppb and 102 ppb). The presence and concentration of the Fe impurities in the three electrolytes is determined by Inductive Coupled Plasma spectroscopy. The rationale for investigating the OER performance of the pure and mixed Ni/Fe catalysts in various NaOH solutions, rather than the widely reported and more expensive KOH, with different Fe impurities was to conclude if the OER activity was comparable to the KOH and if the activity differed between NaOH solutions. A number of the mixed Ni/Fe catalysts in NaOH containing Fe impurities at a concentration of 5 ppb exhibited higher OER activities, with higher Turnover Frequency than the same catalyst in the NaOH solutions containing <1 ppb and 102 ppb and the state if the art RuO2 …