The mechanism of catechol oxidation by soil metal oxides is investigated in a slurry reactor. This abiotic transformation is shown to consist in a three-step process. The first step is a heterogeneous reaction. Catechol undergoes fast, partial oxidation at the expenses of Fe and Mn oxides contained in the soil. In the second step, reduced Fe and Mn are released into the aqueous solution and immediately complexed by catechol. Metal-catecholate complexes are stable at the very low dissolved-oxygen concentration levels attained under nitrogen sparging. The third step is a homogenous reaction. The highly reactive intermediate produced by catechol partial oxidation initiates catechol polymerisation. Under nitrogen sparging, the polymerisation process ends rather rapidly, thus yielding only partial conversion of the phenol and producing low-molecular weight, water-soluble polymers. Further oxidation of the metal-catecholate complexes formed in the second step only occurs under air sparging. Thus, reactive intermediates are formed at much higher concentration levels than those attained when nearly no oxygen is present in solution. The polymerisation proceeds at a much faster rate until, under the experimental conditions adopted, complete catechol conversion is attained and high-molecular-weight, insoluble polymers are produced.