Complex fiber‐matrix interactions occur in the processing of glass fiber reinforced polymeric composites, because of the proprietary, complex composition of commercial sizings applied to the glass fiber surface. Research involving a vinyl ester resin system and three model commercial glass fiber sizings, having varying levels of solubility in the resin, has shown that micro‐dielectrometry can provide important information about interactions and may be useful as a tool in optimizing sizing‐matrix resin interactions. Two distinct types of interactions may be monitored by micro‐dielectrometry: The initial resin swelling of the sizing, as well as the dissolution of the sizing into the resin. An estimate of the times associated with swelling and dissolution of the sizing into the matrix resin can be made from micro‐dielectric measurements to optimize composite processing.