A volumetric measurement of evolved hydrogen is used to quantify rates of corrosion occurring on unpolarized samples of commercially "pure" magnesium immersed in 5% (w/v) aqueous sodium chloride electrolyte. This approach is used to compare rates of uninhibited corrosion with rates occurring in the presence of arsenate and phosphate corrosion inhibitor species dissolved in the experimental electrolyte at concentrations between 10(-4) and 10(-2) mol dm(-3). The effective cathode in commercially pure magnesium comprises a population of micron and submicron size iron-rich particles widely dispersed in the magnesium matrix. It is shown that arsenate, but not phosphate, acts to poison the hydrogen atom recombination reaction as it occurs on the surface of these cathodic particles. It is shown that because hydrogen evolution is the predominant cathodic process the onset of H-atom recombination poisoning results in greatly reduced rates of magnesium corrosion. Additional mechanistic information regarding the effect of phosphate and arsenate corrosion inhibitors is obtained through systematically investigating the effect of solution pH on inhibitor efficiency. (C) 2012 Elsevier Ltd. All rights reserved.