Fluorine has been proposed as a useful geochemical tracer for tracking the transport of volatile elements in subduction zones as it is the last volatile element to be exsolved during magma ascent and crystallization. Accurate determination of the concentration of fluorine in geological fluids under high P-T conditions would greatly further the understanding of its geochemical behavior during aqueous transport and magma genesis in arcs. Mass balance is the most widely used method for calculating fluid compositions in hydrothermal experiments. However, the method is inaccurate when phase compositions and/or proportions have large uncertainties. In the current study, we present an alternative method based on the direct measurement of the concentration of F in fluids by extracting the enclosed liquid within experimental samples, and by performing high-pressure liquid chromatography (HPLC) analysis. We then compared the results of the direct analysis with those obtained by mass-balance calculations. Our experimental data demonstrated that direct analysis yielded more accurate concentration values than mass-balance calculation. The direct method is a useful alternative, when i) the phase assemblage is complex, ii) the total F content or the fluid proportion is low, or iii) when there is significant presence of hydrous melts. Caution should be exercised when applying direct analysis to samples that contained insoluble quench phases and/or were low in fluid. Direct HPLC analysis, combined with an improved extraction method that ensures the separation of quench phase, minerals and melts, could be of great utility for studying water-rock reactions.