The aerobic oxidation of substituted phenols with the catalytic system M(acac)(n)/3 -methylbutanal/O-2 has been investigated. Co(acac)(2) and Mn(acac)(3) promoted the transformation of 2,6-dimethylphenol and 2,6-di-t-butylphenol into their corresponding diphenoquinones and benzoquinones. In the oxidation of 2,3,6-trimethylphenol, the same catalysts yielded 32-34% of the relevant biphenol. Cu(acac)(2) converted 2-naphthol into 1,1'-bi-2-naphthol with 84% yield. Supported Co(II) and Cu(II) complexes have also been used as heterogeneous catalysts for the oxidation of 2,6-di-t-butylphenol and 2-naphthol, respectively.