Oxidation activity and stability of cobalt-Schiff base complex catalyst, Co-sulphosalen, were studied in aqueous alkaline medium. The 4-coordinated Co-sulphosalen was shown to be an active catalyst, with molecular oxygen increasing the initial oxidation rates of both a phenolic (guaiacol) and a non-phenolic (veratryl alcohol) lignin model compounds. Studies with a visible absorption spectrometer showed that 4-coordinated Co-sulphosalen forms a new complex with pyridine, which attaches as axial ligand. The activity of Co-sulphosalen complex was not increased, however, but instead was slightly decreased by the addition of pyridine. With a newly developed HPLC method, it was shown that Co-sulphosalen is not stable but decomposes as a function of time. The decomposition of the catalyst through hydrolysis of its imine structures was independent of the oxidation of the model compound. The decomposition rate of Co-sulphosalen increased with increasing pH and was higher in the absence of oxygen. The presence of pyridine had virtually no effect on the stability of the catalyst. (C) 2002 Elsevier Science B.V. All rights reserved.