Mixtures of the alkynes and acetonitrile were cyclotrimerized with CpCoCOD as catalyst to form pyridine derivatives. The system can be thermally or photochemically activated. The cocyclotrimerization reaction is a pseudo first order process and the photochemically activated reaction at 20°C has a rate constant of 3.66 10 - 3 s - 1 while that of the thermally activated system at 140°C is 4.23 10 - 4 s - 1 . The distribution of the products formed if a mixture of hex-1-yne, phenylacetylene and acetonitrile is used indicates an almost random coordination of the alkynes on the cobalt before the nitrile coordinates. The nitrile cocyclotrimerizes rather with a hex-1-yne containing intermediate than with the phenylacetylene containing intermediate to form pyridine derivatives.