The mechanistic aspects of the dissociation reactions of the molecular ions of isomeric propoxypyridines have been examined with the use of deuterium labelling and tandem mass spectrometry. The ionized 2-and 3-propoxypyridines dissociate predominantly by the loss of propene, whereas the molecular ions of 4-propoxy-and 4-isopropoxypyridine react by the competing losses of a propene molecule and an allyl radical. The loss of propene from the metastable molecular ions of the 2-isomer involves a 1,5-H shift from the 2-position of the propyl group to the ring as evidenced by the labelling experiments. For the metastable molecular ions of 3-propoxypyridine, the results are in agreement with propene loss by a pathway involving formation of a [C 5 H 4 NO β
/(CH 3 ) 2 CH Ο© ] complex which reacts further by proton transfer prior to dissociation. In contrast to these findings, interchange between the hydrogen atoms at the 2-position of the propyl group, and the 3-or 5-position of the pyridine ring, occurs in ionized 4-propoxypyridine. This interchange can precede the formation of a [C 5 H 4 NO β
/(CH 3 ) 2 CH Ο© ] complex and the occurrence of proton transfer within this species. The [C 5 H 5 NO Ο©β
/C 3 H 6 ] complex formed in the proton transfer step may either expel propene or react by hydrogen atom transfer prior to the loss of an allyl radical. For the metastable ions of 4-isopropoxypyridine, interchange between a hydrogen atom from one of the methyl groups of the alkyl chain and the aromatic ring appears to be of minor importance. (Int J Mass Spectrom 199 (2000) 1-16) Β© 2000 Elsevier Science B.V.