Venus water loss is dominated by HCO + dissociative recombination.

Despite its Earth-like size and source material 1,2 , Venus is extremely dry 3,4 , indicating near-total water loss to space by means of hydrogen outflow from an ancient, steam-dominated atmosphere 5,6 . Such hydrodynamic escape likely removed most of an initial Earth-like 3-km global equivalent layer (GEL) of water but cannot deplete the atmosphere to the observed 3-cm GEL because it shuts down below about 10-100 m GEL 5,7 . To complete Venus water loss, and to produce the observed bulk atmospheric enrichment in deuterium of about 120 times Earth 8,9 , nonthermal H escape mechanisms still operating today are required 10,11 . Early studies identified these as resonant charge exchange 12-14 , hot oxygen impact 15,16 and ion outflow 17,18 , establishing a consensus view of H escape 10,19 that has since received only minimal updates 20 . Here we show that this consensus omits the most important present-day H loss process, HCO + dissociative recombination. This process nearly doubles the Venus H escape rate and, consequently, doubles the amount of present-day volcanic water outgassing and/or impactor infall required to maintain a steady-state atmospheric water abundance. These higher loss rates resolve long-standing difficulties in simultaneously explaining the measured abundance and isotope ratio of Venusian water 21,22 and would enable faster desiccation in the wake of speculative late ocean scenarios 23 . Design limitations prevented past Venus missions from measuring both HCO + and the escaping hydrogen produced by its recombination; future spacecraft measurements are imperative.