The determination of tungsten and molybdenum in geological materials by isotope dilution and internal standardisation inductively coupled plasma mass spectrometry (ICP-MS) is reported. The sample is first decomposed by an alkaline fusion, the elements are then leached from the melt as their soluble oxy-anions, and taken through a separation procedure based on selective formation of their oxinate complexes with subsequent adsorption on activated charcoal. Rhenium is used as the internal standard for tungsten and ruthenium as the internal standard for molybdenum. This technique is shown to compensate well for any possible interferences, such as that arising from the presence of sodium. Relative standard deviations of 7 and 5%, and determination limits of 0.07 and 0.08 µg g^–1 are obtained for tungsten and molybdenum, respectively, by isotope dilution ICP-MS. This precision is degraded only slightly when using internal standardisation ICP-MS. Results by both calibration methods are shown to be accurate and comparable to literature values of international reference materials.