An examination is made by mass spectrometry of the negative ions formed when pyridine and methyl-substituted pyridines flow over a heated tungsten filament. For each substrate a small fraction (5-10%) of the negative ions have masses one unit less than the substrate. The relative amounts of these and other ions formed do not vary with filament temperature. Results are interpreted in terms of the formation of a precursor ion and appear to justify the magnetron technique in which the temperature dependence of total negative ion currents is used to determine electron affinities.