Biotite is the dominant ferromagnesian mineral in different granites from the Cierna hora Mountains, in the Western Carpathians (Slovakia). A higher content of Fe³⁺ (up to 20 %) is characteristic for the biotites from I-type Sokol and Sopotnica granitoid bodies in contrast to the biotites from S-type Tahanovce granitoids showing decreased Fe³⁺ amount (around 5 %). The Fe/(Fe+Mg) ratio in biotites from the Sokol and Sopotnica massifs between 0.47 and 0.54 is rather low with respect to that in biotite from the Tahanovce [Fe/(Fe+Mg)=0.55–0.63] and Miklusovce [Fe/(Fe+Mg)=0.73–0.81] granite body. Water fugacities and contents calculated using Wones’ (1981) calibration of biotite stability equation and Burnham’ (1994) water dissolution model yield relatively similar values of 4–5 wt. % in remaining melts at 400 MPa and various levels of f_O2 and activities of annite for magnetite-bearing assemblages. This suggests an effective buffering role of biotite in both oxygen and water fugacities. Comparison of the peraluminosity index (A/CNK) of biotite with the same index in whole-rock shows distinctly higher A/CNK values for biotite indicating its aluminous character and important role as a significant aluminium carrier. The biotite composition indicates that granitoids in the Cierna hora Mts can be primarily derived from the lower crust; their protolith was influenced by mixing and/or assimilation process.