The research results on the lignin oxidation in a flow of water-oxygen mixture at uniform heating (1.5 K/min) of the tubular reactor to 733 K are presented. The temperature dependences of the products yield and composition are obtained. It is shown that 90.3% of carbon is removed from lignin as part of CO2 and CO. The maximum yield of phenols was detected at T < 583 K. Based on time dependences of the reactor wall temperature and power of the resistive heaters, it was revealed that the most intense oxidation occurs at 583-643 K; average heat power in the oxidation of lignin is 0.5 kW. From the comparison of lignin oxidation in the Ar/O-2 flow (thermogravimetric analysis) and in the H2O/O-2 mixture, it follows that in the latter case, the oxidation completes at a temperature by approximate to 120 K lower. According to X-ray diffraction analysis, Na-6(CO3)(SO4)(2), Na2SO4, and Na2CO3 are the major mineral components of residue formed during lignin oxidation by the H2O/O2 mixture. The conversion residue includes the stainless-steel corrosion products.