Abstract Multiple-collector inductively coupled plasma source mass spectrometry (MC-ICPMS) is a new technique of considerable relevance to research in the Earth sciences. The very high ionization efficiency attained in ICP sources is combined with the precision of magnetic sector multiple collector isotopic ratio mass spectrometry to permit very accurate isotopic measurements of elements that are normally difficult to ionize by thermal (surface) means. The addition of a laser facilitates studies for which spatial resolution is required. The precision achieved for isotopic ratios (typically ± 0.001-0.01 %) sets this instrument apart from all other forms of ICPMS. With laser ablation, the sample utilization is much faster than in dynamic SIMS (ion probes), but the greatly enhanced ion beam signals and simultaneous measurement of all masses of interest on multiple stable and linear Faraday detectors result in the most precise isotopic ratios for trace elements yet measured in situ. The multifarious applications of this new technique include in situ Sr and Pb isotopic measurements and Lu-Hf and In-Sn geochronology. Although in its infancy, the techniques can clearly be applied to a variety of problems in low temperature geochemistry, including the dating of ancient mineral deposits, the scales of circulation of crustal fluids and the origins of their dissolved constituents.