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I report the development of a technique for the measurement of mass-dependent Mo isotope ratios. This is used to significantly expand the limited existing dataset of mass-dependent Mo isotope ratios in igneous samples with the aim of better constraining the processes which fractionate Mo isotopes at high-temperatures. Mid-ocean ridge basalts (MORBs) from the Atlantic and Pacific basins have a uniform δ⁹⁸/⁹⁵Mo -0.24 ± 0.02 0/00 (2 s.e.) and provide a new reference value for the upper convecting mantle. This mean value is isotopically lighter than the mean of measured chondrites (Burkhardt et al., 2014) which is seemingly at odds with the prediction that core formation should leave the mantle preferentially enriched in isotopically heavy Mo (Hin et al., 2013). This may be a result of preferential extraction of isotopically heavy Mo to the continental crust. 3.7 Gyr samples from Isua, Greenland have δ⁹⁸/⁹⁵Mo values within error of the MORB value. If this signature is primary, rather than an artefact caused by later disturbance, it could suggest that the upper mantle Mo isotope signature was instead established prior to 3.7 Gyr. Indeed, by measuring more recent altered samples from La Palma and Hawaii, I show that δ⁹⁸/⁹⁵Mo values can be perturbed by alteration. Ocean island basalts show substantial Mo isotope variability δ⁹⁸/⁹⁵Mo -0.59 - 0.03 %0, this study; M. Willbold, unpublished; Lai, 2013). However δ⁹⁸/⁹⁵Mo does not correlate with traditional tracers of recycled material in the mantle such as Pb and Sr isotopes. The observed variability may reflect mixing of an isotopically light subducted component into a deep mantle source which is isotopically heavier than MORB . I also report a technique for measurement of mass-independent Mo isotope ratios with the aim of constraining the influence of the late veneer on mantle Mo, however the reproducibility obtained was insufficient to resolve addition of chondritic material given estimates for the size of the late veneer.