2,6-Dimethylphenol polymerization with catalyst systems based on CuCl/n-butylamine were studied under 10 kg/cm(2) pure oxygen pressure. By addition of sodium iodide, the catalyst performance was dramatically improved and an unusual long induction period was observed. Both the polymer intrinsic viscosity obtained and the induction period increased significantly with the increase of NaI concentration. However, the induction period decreased rapidly with the increase of n-butylamine concentration. UV-VlS absorption spectra showed that the CuCl/n-butylamine complex was converted to CuI/n-butylamine complex after the addition of NaI. The hydration rate of copper halide/n-butylamine complex decreased significantly with the increase of NaI and n-butylamine concentrations. Therefore the increase of polymer intrinsic viscosity and induction period by NaI addition suggest that the polymerization catalyst became more hydrolytically stable and less active at the higher NaI concentration.