Thermal ionization mass spectrometry in the positive or negative ionization mode can be used for precise isotope ratio determinations of most elements. The preferably formed ions are M⁺, M^– and MO_x^–(x= 1–4). Recent interesting developments in this field, which are relevant in geology, are the U/Th and Re/Os dating methods, redeterminations of the relative atomic masses of elements by more precise and more accurate isotope ratio measurements, the determination of isotope variations in geological and cosmic samples and the analysis of trace and ultra-trace amounts of elements and elemental species in the environment. In order to measure accurately a low abundant isotope in addition to a high abundant isotope (up to ratios of 10⁹), thermal ionization mass spectrometers with deceleration lens systems have been constructed which fulfil the required abundance sensitivities. Chromium (III)/chromium(VI) speciation below the µg l^–1 level in different river water samples is reported as an example of the application of positive thermal ionization mass spectrometry in connection with the isotope dilution technique. Rhenium/osmium determinations in iron meteorites of group IIA and IAB are performed by using a corresponding separation procedure with an OsO₄ distillation step and rhenium isolation by anion-exchange chromatography. The isochron for the IIA iron meteorites results in an age of (4.54 ± 0.08)× 10⁹ years. These precise measurements also indicate small deviations of the IAB silicate-rich iron meteorite data from the IIA isochron. Precise boron isotope ratio measurements by negative thermal ionization mass spectrometry are applied to determine anthropogenic influences in groundwater samples.