We evaluate the melt-vapor surface tension (σ) of natural, water-saturated dacite melt at 200 MPa, 950--1055¿C, and 4.8--5.7 wt % H2O. We experimentally determine the critical supersaturation pressure for bubble nucleation as a function of dissolved water and then solve for σ at those conditions using classical nucleation theory. The solutions obtained give dacite melt-vapor surface tensions that vary inversely with dissolved water from 0.042 (¿0.003) J m-2 at 5.7 wt % H2O to 0.060 (¿0.007) J m-2 at 5.2 wt % H2O to 0.073 (¿0.003) J m-2 at 4.8 wt % H2O. Combining our dacite results with data from published hydrous haplogranite and high-silica rhyolite experiments reveals that melt-vapor surface tension also varies inversely with the concentration of mafic melt components (e.g., CaO, FeOtotal, MgO). We develop a thermodynamic context for these observations in which melt-vapor surface tension is represented by a balance of work terms controlled by melt structure. Overall, our results suggest that cooling, crystallization, and vapor exsolution cause systematic changes in σ that should be considered in dynamic modeling of magmatic processes.