Ladenburg found that thermal decomposition of pyridine ethiodide led to both 2- and 4-ethylpyridine. Because no systematic study of this reaction had ever been reported, the present work was undertaken to explore the reaction as a synthetic method, and also, to elucidate the reaction mechanism. To this end a number of alkylpyridine alkiodides have been decomposed and the products identified and analysed quantitatively, Gas chromatography and ion-exchange chromatography were used extensively in the analyses. It was found that the alkyl group entered the pyridine nucleus preferentially at the 2- and the 4- positions. It was also found that alkylpyridine alkiodides rearranged to give, in addition to the nuclear substituted products, substituted in 2- or 4-alkyl sidechains. In the case of the 4-ethylpyridine methiodide rearrangement, 4-iso-propylpyridine is the most important substitution product. Both nuclear and sidechain substitution appear to be intermolecular but while nuclear substitution seems most likely to involve a free radical intermediate, sidechain alkylation appears to involve carbonium ion attack on the deprotonated sidechain of an N-methylalkylpyridinium ion.