A polyurethane (PU) is compared with the polyurethane‐unsaturated polyester (PU‐UPE) hybrid to examine the changes of the resin‐fiber interphase in the presence of a sizing compatible component, UPE, in the matrix resin. The formation of resin‐fiber interphase is critical in structural reaction injection molding (SRIM) and resin transfer molding (RTM) because resin solidification (i.e., gelation and vitrification) occurs in seconds (SRIM) or minutes (RTM). Kinetic (DSC, FTIR spectroscopy), mechanical (single fiber test, fracture surface analysis), and microscopic (birefringence, SEM) studies were used to analyze the interface. Kinetic results indicate poor chemical compatibility of the sizing with PU and increased chemical interactions of the sizing with the PU‐UPE resin. Mechanical and microscopic analyses also point in favor of improved interphasial behavior with the PU‐UPE resin.