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Complexion transitions (CTs) of grain boundaries (GBs) have been a subject of extensive discussions in the last years, but many aspects of this phenomenon are still unclear. Here we studied temperature-induced disordering transitions of GBs in several body-centered cubic metals by means of classical atomistic simulations. Our study shows that gradual heating from room temperature to the melting temperature (Tm) leads to continuous disordering of the GB structure due to spontaneous formation of point defects in all studied metals. This disordering is accompanied by two CTs and exhibits analogies to transitions described by the Berezinskii-Kosterlitz–Thouless-Halperin-Nelson-Young theory. The first CT occurs at temperatures of about 0.7Tm and is characterized by significant changes of mechanical and kinetic properties. The second CT at about 0.9Tm is a premelting transition when the GB order parameter becomes zero.