We developed a novel experimental setup to investigate the transport of Li and B between two melt reservoirs that were connected only via a fluid phase. This was used to simulate fluid exsolution occurring during late-stage pegmatite formation. We found that both Li and B were transported via the fluid, but the observed transport rates were low. It was shown experimentally that the Li isotopes fractionate between melt and fluid with the fluid being preferentially enriched in the heavier ⁷Li.