Dumortierite is described from several occurrences of cordierite-orthoamphibole-bearing rocks in the upper-amphibolite facies area of the Bamble Sector, south Norway. Dumortierite occurs with chlorite, muscovite and quartz in late M₄ alteration zones or aggregates after M₃, peak-metamorphic cordierite and garnet and early M₄ vein-cordierite, and intergrown with or replacing orthoamphibole. Late M₄ P-T conditions, which are interpreted as conditions of dumortierite formation, are estimated from the associated late M₄ kyanite-andalusite-chlorite-quartz assemblage and Mg-Fe exchange geothermometry to be 500 ± 50 °C and 3–4 kbar. Retrogression of M₃ mineral assemblages is initiated by influx of fluids with X_CO2 of 0.3–0.4 during early M₄ followed by more water-rich fluids during late M₄. Late M₄ fluids may show local variations in alkalis and boron. The dumortierites are the most Mg-rich (2.23–3.42 wt. % MgO) ever reported, and contain 0.00–2.05 wt.% TiO₂, 0.00–1.08 wt.% Fe₂O₃, 29.62–32.42 wt.% SiO₂ and 55.20–59.71 wt.% Al₂O₃. Al is the most likely substituent for Si, which shows a minor deficiency at the tetrahedral sites in most dumortierites. The major variations in the mineral chemistry can be described by the coupled substitutions Mg + Ti = 2Al^VI, 3Mg = 2Al^VI and possibly Mg + H = Al^VI. Favoured by low ƒ_O2 prevailing conditions a significant part of total iron in dumortierites at one locality is present as Fe²⁺ giving evidence for the Fe²⁺ + Si^IV = Al^IV + Al^VI tschermakite substitution. FeMg₋₁ substitution is considered to be limited. Ti-rich dumortierites coexist with rutile, ilmenite or titaniferous magnetite. The development of dumortierite from orthoamphibole correlates with an observed decrease of Al, Mg and Na and increase of Si and Fe in orthoamphibole towards dumortierite.