One of the azine ligands in fac-Re(CO)3L2Cl, L = 4-cyanopyridine or quinoline, was found to be more labile than in other related Re(I) complexes. Substitution of 4-cyanopyridine by 4-phenylpyridine in fac-Re(CO)3(4-cyanopyridine)2Cl was successfully applied to the preparation of fac-Re(CO)3(4-cyanopyridine)(4-phenylpyridine)Cl. The kinetics of the mixed-ligand complex formation was investigated and a reaction mechanism is proposed.