The use of a phosphorus containing Ligand as a Ligand for a metathesis catalyst in a catalysed metathesis reaction wherein the phosphorus containing Ligand is a heterocyclic organic compound with a ligating phosphorus atom as an atom in the heterocyclic ring structure of the heterocyclic organic compound. The invention also relates to a metathesis catalyst such a phosphorus containing Ligand and to a metathesis reaction using the catalyst.