Laser ablation ICP MS is an extremely sensitive (yet destructive) analytical technique that allows major and trace elements to be quantified in fluid and melt inclusions as small as 10 μm. Individual inclusions are drilled out of their host mineral by using a laser beam (preferably 193 nm ArF Excimer laser), and the ablated material is continuously transported with an Ar/He gas mixture into the mass spectrometer (Günther and Heinrich, 1999). The resulting transient signals are integrated and compared with signals obtained from standard glasses. The LA ICP MS system at ETH Zürich (Switzerland) allows up to 30 major and trace elements to be measured simultaneously, with detection limits typically being in the order of 10-100 ppm for a medium salinity, 30 μm sized inclusion (Günther et al ., 1998; Audétat et al ., 2000a,b). Main source of analytical uncertainty is the process of internal standardization, for which the absolute concentration of one element has to be estimated by an independent method. In case of fluid inclusions this is done by calculating Na from the total salinity (NaCl equiv ), and correcting this value for other major cations present in solution. The internal standardization of melt inclusions is based on Al, which is determined either by microprobe analysis of re homogenized inclusions, or is estimated from LA ICP MS elemental ratios in conjunction with whole rock geochemical data. Examples of applications include: (i) geochemical evolution of hydrothermal fluids in ore deposits, (ii) metal fractionation between vapor and brine, and (iii) element partitioning between melt and aqueous fluid(s). LA ICP MS analysis of successive generations of fluid inclusions in quartz crystals from the Sn/W mineralized Mole Granite (Australia) demonstrates that ore precipitation was triggered by dilution of the magmatic fluid with meteoric water (Audétat et al ., 1998, 2000a). Partition coefficients determined on pairs of clearly cogenetic vapor and brine inclusions show that Cu, As, Au, B (±Ag, Li, S) fractionate into the vapor phase, and that Na, K, Fe, Mn, Pb, Zn, Sn, Sb, Ba, Rb, Cs, Tl, Bi, W, Cd and Ce fractionate into the brine phase (Heinrich et al ., 1999; Audétat et al ., 2000a). This fractionation behavior points to a dominantly vapor phase origin of Au , As , Cu and Ag rich epithermal ores deposited in near surface environment. Fluid melt partition coefficients determined on natural fluid melt assemblages indicate that high metal contents in the fluid are not the result of high metal concentrations in the melt, but are controlled by fluid phase equilibria. Laser ablation ICP MS is an extremely sensitive (yet destructive) analytical technique that allows major and trace elements to be quantified in fluid and melt inclusions as small as 10 μm. Individual inclusions are drilled out of their host mineral by using a laser beam 193 nm ArF Excimer laser), and the ablated material is continuously transported with an Ar / He gas mixture into the mass spectrometer (Günther and Heinrich, 1999). The resulting transient signals are integrated and compared with signals obtained from standard glasses. The LA ICP MS system at ETH Zurich (Switzerland) allows up to 30 major and trace elements to be measured simultaneously with the detection limits typically being in the order of 10-100 ppm for a medium salinity, 30 μm sized inclusion (Günther et al. 1998; Audétat et al., 2000a, b). Main source of analytical uncertainty is the process of internal standardization, for which the absolute concentration of one element has to be esti mated by an independent method. In case of fluid inclusions is done by calculating Naa from the total salinity (NaCl equiv), and correcting this value for other major cations present in solution. The internal standardization of melt inclusions is based on Al, which is determined either by microprobe analysis of re homogenized inclusions, or is estimated from LA ICP MS elemental ratios in conjunction with whole rock geochemical data. Examples of applications include: (i) geochemical evolution of hydrothermal fluids in ore deposits, (ii) metal fractionation between vapor and brine, and (iii) elemental partitioning between melt and aqueous fluid (s). LA ICP MS analysis of generations generations of fluid inclusions in quartz crystals from the Sn / W mineralized Mole Granite (Australia) demonstrates that ore precipitation was triggered by dilution of the magmatic fluid with meteoric water (Audétat et al., 1998, 2000a). Partition coefficients determined on pairs of clearly cogenetic vapor and brine inclusions show that Cu, As, Au, B (± Ag, Li, S) fractionate into the vapor phase, and that Na, K, Fe, Mn, Pb, Zn, Sn, Sb, Ba, Rb, Cs, Tl, Bi, W, Cd and Ce fractionate into the brine phase (Heinrich et al., 1999; Audétat et al., 2000a). This fractionation behavior points to a dominantly vapor phase origin of Au, As, Cu and Ag rich epithermal ores deposited in near surface environment. Fluid melt partition coefficients determined on natural fluid melt assemblages indicate that high metal contents in the fluid are not the result of high metal concentrations in the melt, but are controlled by fluid phase equilibria.