Cellulose nanocrystals (CNCs) have been studied as compatibilizers for improving the interfacial miscibility of polymer binary blends. Binary blends of water-soluble polymers-poly(vinyl alcohol), poly(ethylene oxide), and polyvinylpyrrolidone-containing different amounts of CNC (16 and 25 wt %) were prepared by a solution casting method. For the first time, a systematic side-by-side comparison between the morphological, thermal, and mechanical properties of the polymer blends reinforced by CNC has been conducted. It has been determined that in the presence of CNC, the degree of crystallinity of the blend components decreased and the miscibility of the blends increased. The blends possess enhanced thermal and mechanical properties as a result of improved miscibility of the blend components.Poly(ethylene oxide) (PEO) and poly(vinyl alcohol) (PVA) are water-soluble semicrystalline synthetic polymers with numerous industrial applications due to their hydrophilic, nontoxic and biocompatible properties, as well as chemical resistance and superb physical properties. 3,4 Polyvinylpyrrolidone (PVP), a water-soluble, nontoxic, nonionic amorphous polymer, has been widely used in nanoparticles synthesis. 5 Combinations of synthetic polymers (PVA, PEO, PVP) with natural ones (cellulose derivatives, chitosan) have been investigated as solid polymer electrolytes, for some biomedical and pharmaceutical applications, and so forth. [6][7][8][9][10][11][12] PVA and PVP are well known to form a thermodynamically miscible system. 13,14 The good miscibility of PVP with PVA can be attributed to hydrogen-bonding intermolecular interactions between the carbonyl and hydroxyl groups of PVP and PVA, respectively, which are confirmed by the data of X-ray photoelectron, infrared, and high-resolution solid-state nuclear magnetic resonance ( 13 C NMR) spectroscopy. 15 Although PVP can interact with poly(ethylene glycol) (PEG) through hydrogen bonding between the terminal hydroxyl groups of the Additional Supporting Information may be found in the online version of this article.