This study explored the direct conversion of black liquor acid sediment (BIAS) into phenolics by hydrothermolysis. Experiments were successfully performed in a batch-type reactor at temperature of 260 to similar to 340 degrees C over 0-120 min. Depending on the reaction conditions, four products (bio-oil phase containing most of the phenolics, aqueous phase, char, and a small amount of gas) were formed and analyzed by element analysis, Fourier transform infrared spectroscopy (FTIR), gas chromatography mass spectrometry (GC-MS), gas chromatography (GC), and pyrolysis-gas chromatography mass spectrometry (Py-GC/MS). The total bio-oil yield was 16-26 wt % based on dry BLAS (D-BLAS), in which the main monomeric compounds were 2-methoxyphenol, 2, 6-dimethoxyphenol, and catechol, with yields of 0.74-1.66, 0.28-4.03, and 0-7.08 wt % based on lignin in the BIAS (BLAS-L), respectively. Analysis of char suggested BLAS-L occurred by hydrothermolytic depolymerization to give aryl-ether linkages (beta-O-4 and alpha-O-4 bonds), hydroxyl and carbonyl groups, and aromatic rings. Finally, the reactions were almost complete because the char had no functional groups of lignin at the optimal reaction condition (300 degrees C and 30 min).