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Ozonolysis is a major tropospheric removal mechanism for unsaturated hydrocarbons and proceeds via “Criegee intermediates”—carbonyl oxides—that play a key role in tropospheric oxidation models. However, until recently no gas-phase Criegee intermediate had been observed, and indirect determinations of their reaction kinetics gave derived rate coefficients spanning orders of magnitude. Here, we report direct photoionization mass spectrometric detection of formaldehyde oxide (CH2OO) as a product of the reaction of CH2I with O2. This reaction enabled direct laboratory determinations of CH2OO kinetics. Upper limits were extracted for reaction rate coefficients with NO and H2O. The CH2OO reactions with SO2 and NO2 proved unexpectedly rapid and imply a substantially greater role of carbonyl oxides in models of tropospheric sulfate and nitrate chemistry than previously assumed.