Abstract: Polymerization of acetylene was tested on olefin metathesis catalysts.Experiments showed that polymerization and cyclotrimerization of acetylene did occur with all the seven catalysts tested. No correlation in catalyst activity and selectivity was found between olefin metathesis and acetylene polymcrization. Ageing of the Bu₄N[Mo(CO)₅Cl]-EtAlCl₂ system showed a tremendous decay in catalytic activity for olefin metathesis, but a minor effect on the rate of polymerzation of acetylene. It seems likely that although metathesis of olefin and polymerization of acetylenc proceed on the same catalyst, the active species is not identical. The fact that the turn over number in acetylene polymerization is very low and the polyacetylene is formed very rapidly implics the presence of very small amount of Mo or W molecules acted as active species for polymerization.