<p>Improved catalyst for the metathesis of olefins comprising a compound of W with a halogen and with a phenoxy group which can carry substituents. It contains 4 halogen atoms and 2 phenoxy groups per atom of W. Preferably, the phenoxy group is halogenated. This catalyst allows the more rapid carrying out of various types of metathesis reactions, when it is used as a co-catalyst in conjunction with organo-metallic compounds, particularly Al or Sn. Among others, it allows the economical preparation of pheromones. It is practically insensitive to traces of water and oxygen.</p>
