Oxidation of kraft black liquor with 14.39% solids content, obtained by cooking of Pinus pinaster wood, was studied by three different processes; chemical oxidation with molecular oxygen at 120-138 degrees C (oxygen partial pressure approximate to 0.15 MPa); biological (enzymatic) oxidation; and combined oxidation (in the sequence : chemical + enzymatic). Lignin degradation was followed by high pressure liquid chromatography (HPLC). Production of low molecular weight compounds, e.g. vanillin, was monitored by gas chromatography (GC). Chemical oxidation produced about 1.2 g of vanillin per dm(3) of liquor (0.904 g per 100 g of solids). Biodegradation products had molecular weights near to that of vanillin and mainly around 100 daltons, but vanillin was not detected. In the combined oxidation process, vanillin was consumed with a concomitant reduction of the HPLC peak.