The volatile output from arc volcanoes is a key parameter for estimating the balance of volatiles within Earth’s dynamics. Such output is typically estimated using data from remote terrestrial and/or space measurements of the SO2 flux from degassing volcanoes. However, active subduction zones are also characterized by intense hydrothermal activity related to degassing and cooling of shallow intrusive bodies. We report data on the chloride flux from hydrothermal systems in the Kuril Islands arc (northwest Pacific) and, assuming that this chloride has a magmatic origin, calculate the corresponding output of H2O, S, and CO2 using average H2O/Cl, S/C, and S/Cl ratios for typical Kuril volcanic vapors. The measured hydrothermal flux of chloride is ∼80% of the measured flux of HCl from persistently degassing volcanoes of the Kuril arc. Therefore, the total volatile output from the arc is almost two times greater than the volcanic flux directly to the atmosphere. This indicates that the proportion of volatiles subducted into the lower mantle is greater than that indicated only by volcanic flux studies.