Stabilized Criegee Intermediates (sCIs) have been identified as oxidants of atmospheric trace gases such as SO₂, NO₂, carboxylic acids or carbonyls. The atmospheric sCI concentrations, and accordingly their importance for trace gas oxidation, are controlled by the rate of the most important loss processes, very likely the unimolecular reactions and the reaction with water vapour (monomer and dimer) ubiquitously present at high concentrations in the troposphere. In this study, the rate coefficients of the unimolecular reaction of the simplest sCI, formaldehyde oxide, CH₂OO, and its bimolecular reaction with the water monomer have been experimentally determined at T = (297 ± 1) K and at atmospheric pressure by using a free-jet flow system. CH₂OO was produced by the reaction of ozone with C₂H₄, and CH₂OO concentrations were probed indirectly by detecting H₂SO₄ after titration with SO₂. Time-resolved experiments yield a rate coefficient of the unimolecular reaction of k_(uni) = (0.19 ± 0.07) s⁻¹, a value that is supported by quantum-chemical and statistical rate theory calculations as well as by additional measurements performed under CH₂OO steady-state conditions. A rate coefficient of k_(CH2OO+H2O) = (3.2 ± 1.2) × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹ has been determined for sufficiently low H₂O concentrations (<10¹⁵ molecule cm⁻³) that allow separation from the CH₂OO reaction with the water dimer. In order to evaluate the accuracy of the experimental approach, the rate coefficients of the reactions with acetaldehyde and acetone were reinvestigated. The obtained rate coefficients k_{(CH2OO+acetald)} = (1.7 ± 0.5) × 10⁻¹² and k_{(CH2OO+acetone)} = (3.4 ± 0.9) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ are in good agreement with literature data.