The enhanced catalytic activity of hydrogen evolution reaction on anodically polarized Mg surfaces, commonly referred to as the Negative Difference Effect, has been the topic of intense investigation in recent years. However, the cause of anodic H-2 remains unclear. To determine the primary source of H-2 evolution on dissolving Mg polarized at anodic potentials, an in-situ scanning vibrating electrode technique (SVET) analysis during galvanostatic polarization, coupled with gravimetric H-2 volume collection and potentiodynamic polarization experiments, were carried out on ultra-high purity Mg (99.9999% Mg). The combination of these methods provided solid evidence that the evolution of hydrogen on dissolving ultra-pure Mg is primarily associated with the regions dominated by the anodic reaction. Although local cathodes corresponding with the dark corrosion film formed during anodic dissolution were revealed by in-situ SVET, they appeared to play a minor role in the process. (C) 2016 Published by Elsevier Ltd.