Coupling laser ablation sampling with MC-ICP-MS detection allows rapid, in situ determination of isotope ratios. However, measured isotope ratios can show large biases relative to the true ratio. We show that, at low laser pulse energies (3 J cm-2), the laser ablation aerosol of copper metal possessed a 65Cu/63Cu ratio that deviated by more than 12 ε units (12 parts per 10,000) from that of the sample. At high pulse energies (9 J cm-2 and above), near isotopically stoichiometric ablation generally occurred. However, even at high pulse energies, on-line laser ablation-MC-ICP-MS Cu isotope ratios showed large biases (15-48 ε units) from the target sample. Filtering larger particles (>0.5 μm) from the ablation aerosol was accompanied by a reduction in signal that was generally much smaller than the associated reduction in volume transport (a factor of 2.8-84 for ablations in Ar), suggesting that volatilisation and ionisation of particles in the IGP was incomplete. Filtered aerosols always yielded isotopic values that were closer to the true value than unfiltered aerosols by an average of 18 ε units. These data suggest that, while significant isotopic fractionation occurred at the ablation site at low laser fluence, the dominant source of isotopic fractionation at high laser fluence was the preferential volatilisation of 63Cu during incomplete vaporisation and ionisation in the ICP of particles greater than approximately 0.5 μm in diameter.