Abstract We present analyses of phyllosilicates, volcanic glass, and a series of experiments and observations on the δ18O of water extracted by rapid thermal pyrolysis at 1450 °C using the TCEA (Thermal Conversion Elemental Analyzer). The study includes the same hydrous glasses that were previously analyzed for their δD values of extracted water (Seligman et al., 2016). We utilize natural phyllosilicates (where water is present as OH– only), natural magmatic and experimentally quenched glasses, (in which water occurs as H 2 O m and OH−), and glasses that underwent low-temperature secondary hydration by meteoric water (almost completely as H 2 O m). Our study documents that: 1) thermal extraction and simultaneous pyrolysis of H 2 O t into CO and H 2 produces little or no exchange (<1‰) in δ18O between hydrogen-bound and silica-bound oxygen; 2) water extracted from different natural phyllosilicates have 103lnα silicate-OH (~δ18O silicate – δ18O OH) values ranging from −3 to 6‰, which is in agreement with previous results using partial fluorination, Density-Functional Theory, and the increment method; 3) water extracted from eruptively or experimentally quenched hydrous magmatic glasses have 103lnα silicate-H2Ot values that decrease with increasing quench temperature from 10‰ (900 °C) to 3‰ (1100 °C); 4) during progressive volcanic degassing, the δ18O of remaining water in glass decrease as the departing H 2 O m is more positive in δ18O, especially at lower temperature; this oxygen isotope trend correlates with δD as the departing H 2 O m is also more positive in δD than the remaining magmatic water. 5) the δ18O of water extracted from glass hydrated at low-temperature by secondary water yields isotopically negative δ18O values that are closer to the δ18O values of local meteoric water than values appropriate for low temperature silicate-water 18O/16O equilibrium; this suggests that upon hydration water simply dissolves into glass as H 2 O; 6) subsequently, secondarily hydrated glasses appear to show an increase in δ18O values that trend towards equilibrium with continued secondary hydration, with time, and at higher temperatures. This research demonstrates that the δ18O of extracted water, despite its 1–2 per mil- analytical precision, provides an isotopic tool for investigations of first order trends of both magmatic degassing, and of secondary hydration by meteoric waters. The trends described here aids hydrogen isotopic variations in the same processeses and combined use of both O and H isotopic variation of water in glass to help fingerprint sources of meteoric water, extent and stages of alteration of glass, and magmatic degassing. Highlights • Water-in-glass δ18O analyses help understanding magmatic degassing and secondary hydration. • Rapid thermal extraction by TCEA carries no or little exchange of water with host glass. • Meteoric water is added in glass as H 2 O with negative (meteoric) δ18O with small exchange with glass. • A mass balance method to resolve δ18O in OH and H 2 O m of water in glass is described. • The δ18O of extracted water from phyllosilicates are comparable to previous and theoretical calculations. [ABSTRACT FROM AUTHOR]