PURPOSE: To obtain a selective hydrogenation catalyst for an olefinic unsaturation in polymers or copolymers by using a palladium salt which is complexed with a specific complexing agent and stabilized by a selected organic stabilizer.

CONSTITUTION: The catalyst precursor comprises a palladium (II) complex salt which is complexed with a complexing agent selected from the group consisting of organic phosphates, dialkylidene acetones and tetraalkyl ammonium hydroxides and is stabilized by an organic stabilizer selected from the group consisting of organic cyanides, ethers, polyethers, organophosphines and organoarsines. The complex salt forms a homogeneous colloidal palladium. (O) catalyst when an olefinic unsaturation in diene polymer and copolymer is selectively hydrogenated.