Two approaches to incorporating black alder (BA) bark extractives-derived polyol into a polyurethane (PU) network were studied. In the first case, fractionation of bark extractives with tetrahydrofuran (THF), focusing on isolating the biomass fraction available for obtaining PU elastomers by casting methods using cyclic ethers as a solvent, was employed. Another approach aimed to obtain liquid bio-polyols that could be suitable for producing rigid PU foams. For this purpose, oven-dried crude BA bark water extracts were liquefied with polyethylene glycol (PEG 400) at temperatures of 130–170 °C. The effect of adding sulfuric acid as a catalyst on biomass processing was studied. Wet chemistry, GC, FTIR spectroscopy, analytical pyrolysis (PyGC/MS/FID) and rheological methods were employed to characterize the obtained polyols and insoluble fractions, enabling an assessment of biomass transformation during processing. The resulting THF-soluble fraction comprised 62% of the BA bark extract, mainly consisting of the xyloside form of the diarylheptanoid compound oregonin, along with oligomeric flavonoids and carbohydrates. The THF-insoluble fraction was most enriched with carbohydrate compounds. Moreover, it was observed that the PEG 400-insoluble fractions were predominantly composed of carbohydrate components. The results indicated that the use of sulfuric acid as a catalyst (1–1.5% of solvent) promotes the complete liquefaction of extractives, enabling biomass content in polyols of up to 15–25%. Surpassing the extract content in the starting suspension up to 30% resulted in incomplete liquefaction of biomass. These findings offer valuable insights into tailoring BA bark extractives as building blocks suitable for obtaining PU materials.