In this work, the two dimensional (2D) pit growth method is applied to Mg thin films to study the anomalous hydrogen evolution phenomenon exhibited by Mg. By analyzing 2D pit growth, it is possible to accurately track the active dissolving surface, which in this case is the pit wall, and to measure the anodic, net and cathodic current densities. It was found that 2D pits in Mg thin films grow under mixed ohmic/activation control at lower applied potentials and under mass transport control at higher potentials. Determination of the exact metal film thickness is critical in making accurate assessments of the current densities, and the issues associated with measuring the metal film thickness are addressed. The anodic current densities for these very thin films are extremely high, on the order of A/cm(2). The hydrogen evolution current densities are also very high in the ohmic/activation control region and consume around half of the anodic current density. The hydrogen evolution current densities were shown to increase with increasing applied potential, which is direct evidence for the anomalous hydrogen evolution behavior. It is clearly shown that the anomalous hydrogen evolution phenomenon happens at the active dissolving sites, which in this case is the 2D pit wall. (C) The Author(s) 2019. Published by ECS.