This work aims at increasing the knowledge about the recovery of aromatic compounds from the lignin fraction of sugarcane bagasse, as well as exploring the possibility to recover these fine chemicals of great concern for many industrial sectors. The major natural products contained in the lignin fraction of such a straw material were extracted in previous work by alkaline hydrolysis using different concentrations of NaOH and amounts of bagasse, and HPLC analyses revealed that the extracts mainly contained p‐coumaric acid, ferulic acid, syringic acid, and vanillin, the first three containing and the last lacking a carboxyl group. All these aromatic compounds have well‐known antioxidant power and are very important in pharmacology. For these reasons, they have been investigated in this study by UV spectrophotometry, with special concern to the pH effect on their spectra and determination of their pKa values. The major natural products contained in the lignin fraction of sugarcane bagasse, specifically the aromatic compounds p‐coumaric acid, ferulic acid, syringic acid, and vanillin with well‐known pharmacologically important antioxidant power, were physicochemically characterized by UV spectrophotometry with special attention to the pH effect on their spectra and determination of their pKa values.