MnOx deposits on porous gas diffusion layer (GDL) substrates for application as catalysts for the oxygen reduction reaction (ORR) in metal–air batteries have been prepared using atomic layer deposition (ALD). The saturation behavior of the bis(ethylcyclopentiadienyl) manganese ((EtCp)2Mn) and H2O ALD system has been investigated. The observed saturation behavior is in disagreement with previous reports in literature. (EtCp)2Mn + H2O depositions exhibited non-ALD behavior, as demonstrated by a high growth per cycle (GPC) at substrate temperatures (Tsub) = 40–50 °C and nonsaturating reactions at Tsub ≥ 60 °C. The introduction of a forming gas (FG) (95% N2 + 5% H2) plasma between the (EtCp)2Mn and H2O doses promoted precursor saturation with a constant GPC (1.15 A/cy) in the Tsub range of 100–200 °C. The effect of saturation behavior on porosity coverage was investigated by coating porous carbon electrodes with ALD MnOx. Energy dispersive X-ray (EDX) spectroscopy, electrochemical surface area measurements, a...