Compositional zoning in plagioclase feldspar is an important archive of the temporal, compositional and thermal evolution of magmatic systems. Modelling diffusion of Mg in plagioclase is commonly used to estimate the timescales of magmatic processes that operate weeks to decades before eruption. To do this, however, requires knowledge of how the anorthite composition influences plagioclase-melt partitioning of Mg. The nature of this relationship has yet to be properly constrained, with inconsistencies between early empirical models, natural observations and more recent thermodynamic models. Here we compiled a data base of 965 calculated plagioclase-melt partition coefficients of Mg using mineral rim-melt pairs from phase equilibria experiments and natural samples. The data set includes a comprehensive range of plagioclase compositions (An 13–An 92), melt compositions (46.5–78.2 wt% SiO 2) and temperatures (720–1430° C). We find that the dependence of the partition coefficient on anorthite content has a major inflection at compositions that correspond to the C 1¯–I 1¯ structural phase transition (An 60 at 1000° C): with a positive dependence in the C 1¯ domain, and a negative dependence in the I 1¯ domain. We also find that this change in partitioning behaviour can explain Mg distributions in natural plagioclases observed in mafic to silicic systems including the Galápagos, Santorini, Krakatau, Toba and Cerro Galán. We have developed a new empirical model for the partitioning of Mg in plagioclase that takes into account this structural transition, in addition to the effects of temperature and melt composition. The dichotomous nature of the partitioning of Mg between plagioclase and melt has significant implications for diffusion chronometry, with the shape of Mg ‘quasi steady state’profiles being controlled by the structural state of plagioclase. Compositional profiles that were previously interpreted to show little diffusion, may be close to a quasi steady state and vice versa. This re-evaluation will be highly influential for understanding thermal states and magmatic histories at a range of tectonic settings.