Process for the telomerization of a conjugated diene,wherein the conjugated diene is reacted with a compound containing an active hydrogen atom and having a formula R'--H in the presence of a telomerization catalyst based on: (a) a source of group VIII metal, (b) a bidentate ligand wherein the bidentate ligand has the general formula I R.sup.1R.sup.2M.sup.1--R--M.sup.2R.sup.3R.sup.4 (I) wherein M.sup.1 and M.sup.2 are independently P, As or Sb; R.sup.1, R.sup.2, R.sup.3 and R.sup.4 independently represent a monovalent aliphatic group; or R.sup.1, R.sup.2 and M.sup.1 together and/or R.sup.3, R.sup.4 and M.sup.2 together independently represent an optionally substituted aliphatic cyclic group with at least 5 ring atoms, of which one is the M.sup.1 or M.sup.2 atom, respectively; R represents a bivalent organic bridging group; and novel bidentate diphosphines which can be used in this process.