Molybdenum isotopic compositions are precisely determined by MC-ICP-MS measurements using a Mo double spike. The double spike is added prior to chemical purification, so that laboratory and instrumental mass fractionations are separated from natural mass-dependent fractionation. Fractionation is determined on four Mo mass ratios, providing an internal consistency check. The external standard reproducibility is at 0.06 per mil on the 98Mo/95Mo ratio (2 standard deviation). Using a normal microconcentric nebuliser with a cyclonic spray chamber, the minimum quantity of Mo is ~1 g for high-precision results. A hydrothermal molybdenite shows fractionation of -0.3 per mil on the 98Mo/95Mo ratio relative to our standard (Johnson Matthey, 1000 g/mL (?0.3%) ICP standard solution, lot 602332B). Fine-grained sediments show fractionation of 0.1 and -0.3 per mil on the 98Mo/95Mo ratio. The observed Mo isotope fractionation is small but resolvable with the presented high-resolution technique.