We investigated the relative reactivity of stabilized CH2OO, produced by ethene ozonolysis, toward acetic acid and water vapor at a temperature of 298 +/- 2 K and atmospheric pressure. Hydroperoxymethyl acetate produced through the reaction between stabilized CH2OO and acetic acid was monitored using a chemical ionization mass spectrometer as a function of the acetic acid concentration at different relative humidities. The rate of the reaction between CH2OO and water vapor depended quadratically on the water vapor concentration, suggesting that CH2OO reacted with water dimers in preference to water monomers. We obtained the bimolecular rate constant for the reaction between CH2OO and water dirtier relative to the rate constant for the reaction between CH2OO and acetic acid, k(3)/k(1), of (6.3 +/- 0.4) X 10(-2). The k(3) value of (8.2 +/- 0.8) x 10(-12) cm(3) molecule(-1) s(-1) was derived by combining with a k(1) value of (1.3 +/- 0.1) x 10(-10) cm(3) molecule(-1) s(-1), which has been previously reported by direct kinetic studies. The k(3) value thus obtained is consistent with the absolute rate constants measured directly, suggesting that the reactivity of CH2OO is irrespective of the CH2OO generation method, namely, ethene ozonolysis or diiodomethane photolysis. We indirectly determined the yield of stabilized CH2OO from the ozonolysis of ethene of 0.59 +/- 0.17 and 0.55 +/- 0.16 under dry and humid (relative humidity 23-24%) conditions, respectively, suggesting that the yield is independent of the water vapor concentration. Our results suggest that hydroperoxymethyl acetate is the sole product of the reaction between stabilized CH2OO and acetic acid. The approach presented here can likely be extended to studies of the reactivities of more complicated and atmospherically relevant stabilized Criegee intermediates.