Tandem mass spectrometric measurements of naturally occurring tropospheric ions have identified pyridine, methylpyridine, and ethylpyridine as the parent neutral species responsible for ions observed at mass 80, 94, and 108 respectively. High-resolution mass spectra of natural ions observed over an extended mass range are also shown for two field study locations. At one site (Sapelo Island, Georgia), pyridine and its homologues dominated the positive spectrum. At Mount Evans, Colorado, the second site, pyridine along with a large number of more massive and presumably very alkali species were observed to dominate the positive spectrum. In addition, X ray-produced ion spectra were also measured and used to make rough estimates of the pyridine concentrations at the Sapelo site (~10 ppt) and the total concentration of alkali compounds, excluding NH3, at Mount Evans (~0.5 ppb) ¿ American Geophysical Union 1988