Blends of poly(ethylene oxide) with organosolv lignin (Alcell) were prepared by thermal blending. Excellent fiber spinning was achieved over the entire blend ratio. The good thermal properties of the Alcell lignin arise from its unique chemical structure. HMQC 2D NMR analysis revealed the presence of alkoxyl chains at the Calpha and Cgamma positions of the Alcell lignin side chain structure acting as internal plasticizers and enhancing the thermal mobility of the lignin. The addition of a small amount of Alcell lignin to PEO resulted in an increase of the PEO crystalline domain size. However, both PEO crystallinity and crystalline domain size decreased with lignin incorporation beyond 25 wt %. A negative polymer-polymer interaction energy density "B" was calculated on the basis of the melting point depression of PEO and a negative deviation of T-g from the weighted average values observed. Good prediction of the T-g-composition behavior was obtained indicating the presence of favorable interactions between blend components. FT-IR analysis revealed the formation of a strong hydrogen-bonding system between Alcell lignin and PEO, supporting that hydrogen-bonding interactions are an important factor in the formation of miscible lignin-based polymer blends.