The principles and methods of mass spectrometric analysis as far as they are applicable to mineralogy are described. Some recent investigations have been reviewed, covering essentially age determinations on various minerals, the fission of uranium occurring in nature, and anomalous isotope ratios of argon from uranium minerals. A new set of nomographs for age determinations on the ratios of U/Pb, Th/Pb, and Rb/Sr for a varying isotopic composition of the daughter element has been included here.