For CO₂ removal from flue gases containing SO₂ at thermal power plants, secondary alkanolamine with high molecular weight (abb. HIA) was selected as an absorbent because of low utility consumption. SO₂ forms sulfite salt in absorbent and it is oxidized with O₂ to form the sulfate. These salts called Heat Stable Salts (HSS) reduce CO₂ removal ratio. On the other hand, the sulfite consumes O₂ in the absorbent and the resistance to oxidation of HIA can be expected. We estimated the relationship between concentration of HSS and CO₂ removal ratio, and the resistance to oxidation of HIA absorbent by the presence of SO₂ in treated gas. Based on these results, pilot testing (flue gas capacity: 1, 000 Nm³/h, CO₂ removal ratio: 90%) was carried using coal oil mixture-fired power plant flue gas for continuous 2, 000 hours (accumulating time of 2, 441 h) to evaluate the resistance to oxidation and regeneration heat of HIA.<BR>Oxalic acid by oxidative degradation was accumulated 0.0095% after completion of pilot testing, and its amount was 0.1 times as low as that of LNG-fired flue gas. Furthermore, four kinds of cyclic amines were detected by GC-MS analyses and three of them were compounds by oxidative degradation. Their amounts of the mixture were spectrophotometrically determined to be 0.19% based on a reasonable assumption of the average molar absorptivity of the mixture. Average oxidation rate of the mixture was estimated 8.8x10^-5%/h. This was 0.4% of the total loss of HIA.<BR>Regeneration heat of HIA was 20% as low as that of the conventional absorbent even in the case of accumulation of HSS up to 2% as the reduced concentration of H₂SO₄.