An estimate of TiO2 activity () is necessary for the application of trace-element thermobarometry of magmatic systems where melts are typically undersaturated with respect to rutile/anatase. Experiments were performed in the system SiO2-Na2O-TiO2 to develop two independent methods of estimating —one based on the commonly applied rutile-saturation technique and another utilizing a novel Ti-in-tridymite thermometer. It is demonstrated that the rutile-saturation model can lead to an overestimate of relative to TiO2 activity calculated using the solubility of Ti in tridymite (SiO2) coexisting with rutile. Overestimation via the rutile-saturation technique is due to variations in the solubility mechanisms of Ti in the melt phase as a function of Ti content. In natural systems, overestimates of will lead to an underestimation of crystallization temperatures by Ti-based trace-element thermobarometers. Although this study is not directly applicable to natural systems, it lays the groundwork for future research on natural composition magmas to constrain TiO2 activity in melts.