The perovskite-type oxide catalyst LaCoO3, prepared by the sol-gel method, was tested for catalytic wet oxidation (CWAO) of lignin to aromatic aldehydes. The lignin conversion and yield of each aromatic aldehyde were significantly enhanced by the catalytic process, compared to the noncatalytic process. A mechanism involving the reaction of lignin molecules with adsorbed oxygen surface sites, Co(surf)3+O2-, was proposed on the basis of experimental observations, yielding the cycle of Co-(surf)(3+) -> Co-(surf)(2+) -> Co(surf)3+O2- -> Co-(surf)(3+). The formation rates of the intermediates quinonemethide and hydroperoxide were the rate-determining steps. The activity and perovskite-type structure of the LaCoO3 catalyst did not undergo any obvious changes after the five successive reactions.