Ammonium fixed in micas of metamorphic rocks is a sensitive indicator both of organic–inorganic interactions during diagenesis as well as of the devolatilization history and fluid/rock interaction during metamorphism. In this study, a collection of geochemically well-characterized biotite separates from a series of graphite-bearing Paleozoic greenschist- to upper amphibolite-facies metapelites, western Maine, USA, were analyzed for ammonium nitrogen (NH4+-N) contents and isotopic composition (δ 15 N NH4 ) using the HF-digestion distillation technique followed by the EA-IRMS technique. Biotite separates, sampled from 9 individual metamorphic zones, contain 3000 to 100ppm NH4+-N with a wide range in δ 15 N from +1.6‰ to +9.1‰. Average NH4+-N contents in biotite show a distinct decrease from about 2750ppm for the lowest metamorphic grade (∼500°C) down to 218ppm for the highest metamorphic grade (∼685°C). Decreasing abundances in NH4+ are inversely correlated in a linear fashion with increasing K + in biotite as a function of metamorphic grade and are interpreted as a devolatilization effect. Despite expected increasing δ 15 N NH4 values in biotite with nitrogen loss, a significant decrease from the Garnet Zones to the Staurolite Zones was found, followed by an increase to the Sillimanite Zones. This pattern for δ 15 N NH4 values in biotite inversely correlates with Mg/(Mg+Fe) ratios in biotite and is discussed in the framework of isotopic fractionation due to different exchange processes between NH4+-NH3 or NH4+-N2, reflecting devolatilization history and redox conditions during metamorphism.