The structural changes of organosolv lignin from E. globulus processed with alkaline catalysts (NaOH and KOH) have been evaluated. The aim was to study the effect on lignin structure as well as to optimize conditions necessary for an increase in hydroxyl phenolic group content as a main response to base catalyzed depolymerization (BCD). A central composite factorial experimental design was used to optimize conditions, and total hydroxyl phenolic groups (from syringyl and guaiacyl units) were determined by 31P-NMR as a response. The maximal yield of hydroxyl phenolic groups reached values of 5270 and 3970 µmol per gram of lignin-1 when NaOH and KOH, were used, respectively, while for the control lignin, the maximal yield of hydroxyl phenolic groups was 2830 µmol per gram of lignin-1. The lignin obtained under these conditions was analyzed via SEC, DSC, and TGA. The results indicated that molecular weight (Mw) and glass transition temperature (Tg) for lignin-NaOH were lower compared to those of lignin-KOH and lignin-control. The weight loss showed that both lignin derivatives have greater thermal stability at high temperatures than does lignin-control after heating at 550 °C. Finally, in conclusion, the results show higher modifications in the structure of lignin as a result of the oxidation of lignin, mainly by cleavage of the ß-O-4 ether bond and variations in thermochemical properties when NaOH is used as an alkaline catalyst.