The interdiffusion between a low and a high viscosity epoxy resin was studied on model systems representing the novel composite manufacturing process called "Same Qualified-Resin Transfer Molding" (SQ-RTM). Neat resin model systems were first characterized after curing by Raman spectroscopy, energy dispersive X-ray microscopy, and nano-indentation, all methods reveal an interdiffusion distance of about 700-900 mu m. Transmission electron microscopy further revealed a complex morphological gradient structure in the interdiffusion zone. The interdiffusion distance was about 800 mu m in the absence of carbon fibers, reducing down to 500 mu m when the viscous resin was replaced by the corresponding prepreg, due to the geometrical constraints imposed by the fibers. Moreover, some asymmetry was observed in the interdiffusion profile because of the viscosity difference between the resins. The results obtained on the model systems were found to match very well the interdiffusion profiles generated in real SQ-RTM composites processed with the same combination of resins. (C) 2016 Society of Plastics Engineers