Composition measurements of atmospheric positive ions were made in the middle troposphere (3000--6000 m) above central Europe using an improved aircraft-borne mass spectrometer. The by far most abundant ''core ion'' species was found to be protonated pyridine (H+C5H5N) whose identification is strongly supported by in-flight ECD (electric field induced collisional dissociation) studies of this ion resulting in fragment ions which are characteristic for H+C5H5N. ACIMS (active chemical ionisation mass spectrometry) studies carried out on the same flights independently reveal the presence of a trace gas consistent with pyridine posessing a mole fraction of 1.6 pptv (parts per trillion by volume) at about 4000 m altitude. The formation mechanism of H+C5H5N is briefly discussed. ¿ American Geophysical Union 1990