The magnetic behavior of some biotite samples, from volcanic rocks of W. Thrace area, N.E. Greece, was studied in the temperature range from 85 to 270°K and compared with the chemical composition. The magnetization of the samples depends linearly on the applied field, and the magnetic susceptibility obeys the Curie law, showing a typical paramagnetic behavior which is due to Fe2+, Fe3+ and Mn2+ The effective magnetic moment was calculated from the structural formulas of the samples and deviation from the experimental values is discussed.