Carbonyl oxides, or Criegee intermediates, are important transient species formed in the reactions of unsaturated hydrocarbons with ozone. Although direct detection of Criegee intermediates has recently been realized, the main atmospheric sink of Criegee intermediates remains unclear. We report ultraviolet absorption spectroscopic measurements of the lifetime of the simplest Criegee intermediate, CH2OO, at various relative humidity levels up to 85% at 298 kelvin. An extremely fast decay rate of CH2OO was observed at high humidity. The observed quadratic dependence of the decay rate on water concentration implied a predominant reaction with water dimer. On the basis of the water dimer equilibrium constant, the effective rate coefficient of the CH2OO + (H2O)2 reaction was determined to be 6.5 (±0.8) × 10(-12) cubic centimeters per second. This work would help modelers to better constrain the atmospheric concentrations of CH2OO.