The infrared spectra of 27 biotite samples, collected from various granitic bodies of southeastern China including Nanling and middle and lower Yangtse Valley, were obtained on four different types of infrared spectrometers. The res- ults are well correlated. According to Fosters classification all the samples can be divided info two sexier-Mg-biotite and Fe2+ -biotite. The MFA values (Mg/ Mg+Fe23 +Fe3+ +Mn+ivAl) of the Mg-biotite range from 0.01 to 0.20, while those of the Fe2+ -biotite from 0.4 to 1.40. The shift of some characteristic ab- sorption bands in consequence of their chemical compositions is determined and discussed. The substitution of the major cations for each other is also discussed and referd to their crystal structure. The investigation shows that it is possible to indicate and estimate the ca- tion contents in the octahedral sites and the ratio of Al/ Si in the tetrahedral sites of biotite by means of the infrared spectrometry.