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 R1R2M1—R—M2R3R4  (I) wherein M1 and M2 are independently P, As or Sb; R1, R2, R3 and R4 independently represent a monovalent aliphatic group; or R1, R2 and M1 together and/or R3, R4 and M2 together independently represent an optionally substituted aliphatic cyclic group with at least 5 ring atoms, of which one is the M1 or M2 atom, respectively; R represents a bivalent organic bridging group; and novel bidentate diphosphines which can be used in this process.