The oxidation of synthetic ferruginous biotite in hydrothermal systems was studied for two ideal compositions, annite, KFe3AlSi3O10(OH)2, and siderophyllite, KFe2.5Al2Si2.5O10(OH)2. These biotite samples were annealed in Shaw-type vessels under controlled fH2, at a total pressure of 1 kbar. The ratio of Fe2+ to total Fe, determined by wet-chemical analysis, is directly proportional to the fH2 regardless of temperature variations. For fH2 values of <1 bar to 102 bars, this ratio varies from 0.72 to 0.90 for annite and from 0.76 to 0.96 for siderophyllite. The H contents of the biotite do not vary with Fe3+ content for either composition and result in 94 and 100% of the OH sites being occupied in annite and siderophyllite, respectively. A consistent decrease in total octahedral cation content with increasing Fe3+ content for both annite and siderophyllite suggests that the Fe-vacancy substitution 3Fe2+ ⇌ 2Fe3+ + Vac plays a dominant role in the oxidation of these synthetic biotite samples.