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Two-dimensional (2D) crystals are promising materials for developing future nano-enabled technologies^{, , , , –}. The cleavage of weak, interlayer van der Waals bonds in layered bulk crystals enables the production of high-quality 2D, atomically thin monolayers^{, , –}. Nonetheless, as earth-abundant compounds, metal oxides are rarely accessible as pure and fully stoichiometric monolayers owing to their ion-stabilized ‘lamellar’ bulk structure^{, , –}. Here, we report the discovery of a layered planar hexagonal phase of oxides from elements across the transition metals, post-transition metals, lanthanides and metalloids, derived from strictly controlled oxidation at the metal–gas interface. The highly crystalline monolayers, without the support of ionic dopants or vacancies, can easily be mechanically exfoliated by stamping them onto substrates. Monolayer and few-layered hexagonal TiO₂ are characterized as examples …