Determination of 6 Li/ 7 Li ratios using Li 2 BO 2 + ions analysed by thermal ionisation mass spectrometry with ion counting has been investigated by numerical simulation and experiment. It is possible to solve for one or more of the 6 Li/ 7 Li ratio, 10 B/ 11 B ratio and fractionation if one or more of the ratios of the 54, 55, 56, 57, 58 and 59 masses of Li 2 BO 2 + are measured. Monte Carlo simulation of data sets suggests that within-sample precision on measurement of Li 2 BO 2 + mass ratios is largely limited by counting statistics and should result in precision as good as 0.25‰ (1σ) on the 6 Li/ 7 Li ratio. However repeatability of 6 Li/ 7 Li and 10 B/ 11 B ratios gives precisions no better than approximately ±3‰ (1σ) and the probable cause is fractionation of the 6 Li/ 7 Li ratio on the filament due to the large Li + ion beam produced during analysis of Li 2 BO 2 + . Plots of variation of sample fractionation with time confirms that fractionation is primarily controlled by varying 6 Li/ 7 Li ratio rather than varying fractionation of Li 2 BO 2 + or 10 B/ 11 B ratio.