Functionalization of bio-based oligomers is a relevant approach in order to allow these materials to replace more conventional and nonbiodegradable polymers in a wider range of applications. This article will report on the hydroxylation of isosorbide-based ethersulfone oligomers. The hydroxy end-group was controlled by the variation of polycondensation parameters. Comparative study was conducted to assess the effect of isosorbide excess and the monomer concentration on structural composition and molecular weights of produced oligomers. Depending on reaction conditions, various products were synthesized, mainly alpha,omega-dihydroxyethersulfone oligomers. The characterization of the synthesized polyethersulfone was demonstrated by MALDI-TOF, NMR spectroscopy, differential scanning calorimetry, and size exclusion chromatography.