We have developed a non-radioactive chemical ionization mass spectrometry method using acetic acid-acetate cluster as the reagent ion. By electron impact onto an acetic anhydride-Ar gas mixture, a strong and single ion peak of the acetic acidacetate cluster was observed. We demonstrated that the reagent ion can ionize carboxylic acids (RC(O)OH) and hydroperoxides (ROOH) produced in ethylene ozonolysis through the formation of adduct ions of these species and acetate. This technique could be used for the detection of various organic hydroperoxides as well as organic and inorganic acids in the atmosphere.