This work compares solvent-cast poly(ethylene oxide) (PEO) nanocomposites reinforced with cellulose nanocrystals (CNCs) and fumed silica. Mechanical properties and crystallization behavior were investigated over a range of polymer molecular weights (10 000-100 000 g/mol) and particle loadings (1-10 wt %). Polymer adsorption to CNCs and fumed silica was found to alter PEO undercooling and inhibit crystal nucleation. Atomic force microscopy revealed PEO adsorbs to CNCs in a shish-kebab morphology that is readily incorporated into the crystalline domains of the polymer. Tensile testing and nanoindentation showed that Young's modulus increased by more than 60% for CNC reinforced nanocomposites, and that the Halpin-Kardos model could effectively describe the mechanical properties. Fumed silica reinforced nanocomposites were fit to the Guth-Gold micromechanical model using effective particle volume fractions. Although only solvent-cast nanocomposites were investigated, this work provides new insight into the interactions that control dispersion, crystallization, and mechanical reinforcement.