The aim of this study was to demonstrate the application of dielectric spectroscopy to composite material characterization during processing. Therefore, the microdroplet pullout test was used to follow the mechanical interfacial properties during processing. The results showed that interfacial shear stress strongly depends on the pressure, temperature, and process time at resin deposition. These observations were understood with dielectric spectroscopy. Literature data were first applied to our system. Results based on the ionic conductivity signal showed the existence of a mutual diffusion mechanism at the interface The mechanism rate constant depends strongly on the temperature, pressure, and process time. In a second part, we developed a way to follow the penetration depth with the permittivity signal. This approach is based on electric field line exploitation. The results highlighted the same observations. In conclusion, it appears that the adhesion of matrix-fiber is increased by composite processing at the monomeric state of the resin. Moreover, this study shows that this improvement is a consequence of a mutual diffusion mechanism at the interface.