The mechanism of the ring closure of [2-(1-pyrrolidinyl)phenylmethylene]-propanedinitrile (2a) to l,2,3,3a,4,5-hexahydropyrrolo[1,2-a]quinoline-4,4-dicarbonitrile (3a) has been studied by kinetic measurements using 1H-NMR spectroscopy. It could be shown that the rate-determining step consists of an intramolecular 1,5 hydrogen transfer, which is accompanied by charge separation within the- molecule. The calculated (AM1) and experimental (X-ray) molecular structure of 2a are in fairly good agreement. In the ground state geometry a 1,5 hydrogen transfer will most likely take place suprafacially. Subsequent rotation of the former vinyl group and C-C-bond formation, leading to a six-membered ring, also take place in a stereochemically defined way.