The titanium accessory mineral titanite is increasingly used for geochronology and geothermometry. Petrologic evidence suggests that titanite can precipitate from hydrothermal fluids. However, the aqueous solubility of titanite and its dependence on temperature and fluid composition are poorly known. We propose to use isotope dilution and conventional weight loss methods to measure the aqueous solubility of titanite over a range of temperatures and fluid compositions and then generalize the results by fitting thermodynamic parameters and using them in geochemical models. Through experiments and modeling we plan to identify the conditions under which titanite can precipitate from hydrothermal fluids.