Interfacial shear strength (IFSS) of carbon fibers with vinyl ester resin was investigated as a function of the structure of the polymer coating on carbon fibers via microdroplet tests. For coating carbon fibers, high-performance polymers such as poly(arylene ether phosphine oxide) (PEPO), Udel(R) P-1700, and Ultem(R) 1000, water-soluble poly(hydroxy ether ethanol amine) (PHEA), water-dispersed carboxy-modified poly(hydroxy ether) (C-PHE), and water-insoluble poly(hydroxy ether) (PHE) were utilized. Adhesion of polymers to carbon fibers was also evaluated and the failure surface of the fibers was analyzed by SEM to understand the adhesion mechanism. Diffusion between polymers and vinyl ester resins was investigated and the solubility parameters of the polymers were calculated, with the results being correlated to the IFSS. A highly enhanced IFSS was obtained with PEPO coating, while marginally improved IFSS resulted from PHE, Udel(R), and C-PHE coatings. However, PHEA and Ultem(R) coatings showed no improvement. Such results were attributed to the extent of solubility and/or miscibility of polymer coatings in vinyl ester resin, with better solubility and miscibility leading to a higher IFSS.