The effects of Skydrol 500B-4 upon the physical and mechanical properties of the final network structure of a DDS-crosslinked TGDDM resin were investigated using conventional thermal analysis techniques. The chemical system studied was I.C.I’s Fiberite 934 epoxy resin. The characteristic thermal transitions associated with epoxy networks were correlated with the structure, employing dynamic mechanical thermal analysis (DMTA) and dielectric thermal analysis (DETA). A decrease in the temperature location of the glass/rubber and vitreous transitions, accompanied by a reduction in the peak height and area beneath the beta transition, were observed in cured systems incorporating the hydraulic fluid. An increase in the elastic modulus was also noted in the contaminated networks. Solvent extraction, using a Soxhlet apparatus, was performed to further characterize the role played by the Skydrol.