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Chemical ionization mass spectra of a number of nucleosides, their analogs, and purine and pyrimidine bases have been studied usingCH4, Í-C4H10, NH3, MeNH2, Me2NH, and Me2N reagent gases at approximately 0.4 Torr pressure. In addition to protonated molecular species, the principal ion product from nucleosides was base (b)+ 2H and, in lesser amounts, b+ 30 and b+ 44 in analogy to electron ionization spectra. Based in part on isotopic labeling, the bH2 species was concluded to arise preferentially by initial base protonation followed by transfer of hydrogen from 0-2'with cleavage of the glycosidic bond. Proton affinity limits of 13 bases and 8 nucleosides were estimated by the mass spectrometric bracketing technique based on occurrence of proton transfer from reagent gases of known proton affinity. Orders of basicity obtained for the common bases and nucleosides were cytosine, adenine, guanine> …