Fiber sizing can improve the performance of fiber-reinforced polymer composites. The focus of this work was to determine if the improvement in performance could be ascertained from a micromechanical test for interfacial adhesion on as-processed materials under hygrothermal exposure. Three types of sizings were examined: a carboxyl modified poly(hydroxyether), that is identified as low spread phenoxy (LSP), a poly(vinylpyrrolidone) (PVP) sample and a standard industrial sizing (G'). A nanoindenter was effectively used to obtain interfacial shear strength (IFSS) using a modified micro-indentation technique. The results showed that LSP outperforms the PVP and G' materials in bulk composite properties, but showed equivalent interfacial shear strength to G' and experienced hygrothermal degradation in interfacial adhesion that the PVP did not. The LSP composite loses 10% of its original interfacial shear strength after 576 h, while for PVP composite it improves by 25%. The tensile strengths for LSP and PVP composites decrease by 7% and 10%, respectively, at 576 h of hygrothermal exposure. The relationship between tensile strength and interfacial adhesion proved to be weak, but processing defects and other failure processes showed a strong influence of interfacial adhesion on tensile strength of compsites.