A chemical ionization mass spectrometer (CIMS) was used to study reactions of protonated ethanol clusters (C2H5OH)nH+ with dimethylsulfoxide (DMSO), dimethylsulfone (DMSO2), ammonia (NH3), and a series of nonmethane hydrocarbons (NMHCs) and volatile organic compounds (VOCs). The reactivity of the (C2H5OH)nH+ cluster ions is a function of cluster size with reactivity decreasing as cluster size increases. Ethanol cluster ion distributions that formed at atmospheric pressure from 24 ppbv, 900 ppmv, and 1% ethanol/N2 gas mixtures were studied. Small (C2H5OH)nH+ clusters, those formed using the 24 ppbv ethanol/N2 mixture, react at or near the collisional rate with DMSO, NH3, acetone, and methyl vinyl ketone (MVK). The effective ion molecule rate coefficients are 1.8 ¿ 10-9, 1.5 ¿ 10-9, 1.0 ¿ 10-9, and 1.6 ¿ 10-9 cm3 molecule-1 s-1, respectively. Only DMSO and NH3 react efficiently with the two larger (C2H5OH)nH+ cluster ion distributions studied. The effective rate coefficients for DMSO and NH3 with the 900 ppmv ethanol cluster ion distribution are 1.5 ¿ 10-9 and 0.7 ¿ 10-9 cm3 molecule-1 s-1, respectively. The effective rate coefficient for DMSO with the 1% ethanol/N2 mixture is 0.35 ¿ 10-9 cm3 molecule-1 s-1, while NH3 reaches equilibrium with this cluster ion distribution. Experiments show that large (C2H5OH)nH+ ion clusters must be used at relative humidities greater than 50% at 20¿C to prevent formation of and subsequent interferences from H3O+ ions. These results demonstrate that the (C2H5OH)nH+ ion chemistry can selectively detect DMSO and NH3 under most ambient atmospheric conditions with high sensitivity.