Abstract In this study the zinc and magnesium diffusion in ZnMg thin PVD films is examined and linked to the crash adhesion performance of the coating-substrate system. The investigation is conducted at 175 °C because this temperature conforms to both the temperature range during production of the films and the post process steps at automotive customers. The results confirm that zinc is the faster diffusive specie and although the movement of zinc results in a phase transformation, first from magnesium into MgZn2 and later from MgZn2 into Mg2Zn11 this phase change does not seem to alter the crash adhesion performance of the coating substrate system. Additionally, the magnesium seems to diffuse along grain boundaries towards the steel substrate interface. This segregation of magnesium and the enrichment of magnesium at the interface results in a deterioration of the crash adhesion resistance. The time until the samples fail the crash adhesion test is determined by the distance the magnesium has to travel from the Mg containing layer to the steel-substrate interface.