The feasibility of using OH- as reactant ion in negative chemical ionization mass spectrometry (NCI) has been demonstrated.The OH- ion is formed by electron bombardment of a mixture of N2O and either Hz or CH4. Dissociative electron capture by NzO produces 0 - a , which abstracts a hydrogen atom from H2 or CH4 to form OH-. The spectra of OH- with a variety of organic compounds have been measured and reactions have been postulated for the production of many of the observed ions. For carboxylic acids, amino acids, alcohols, ketones, and esters proton abstraction is the major reaction, and abundant (M - 1)- ions are observed. For ethers and amines, on the contrary, extensive fragmentation and no (M - 1)- ion formation is observed. For weak acids such as substituted aromatics, alkynes, and alkenes, (M - 1)- ions are formed, which appear to react with the N20 present to form as major ions (M - H + NzO)- and (M - H + N2O - H>O)-. We suggest that the (M - 1 + N2O - H2O)- ion is a substituted pyrazole anion. No reactions are observed between OH- and benzene, cycloalkanes, and alkanes. Spectra of organic compounds using N20 as reactant gas have been determined, and the reactions are distinctly different from those using N20/CH4 or NzO/H2 as reactant gas. Spectra of mixtures of N2O with C2H6, C ~ H Ban, d i-C4HIO, respectively, have been measured. In all cases OH- is formed, but in C3Hg/N20 and i-C4H10/N20 mixtures abundant ions are also found at m/e 67 and 81, respectively. We suggest that these ions are produced by the reaction of 0-. with the hydrocarbon to form (M - 1)- ions, which in turn react with NzO to give pyrazole or methylpyrazole anions, respectively.