Changes in microstructure and mechanical properties are investigated as a function of epoxy-amine stoichiometry. The epoxy-amine system studied exhibits a two-phase structure consisting of a hard microgel phase and a dispersed phase of soft, unreacted and/or partially reacted material. The size distribution of the microgel regions tends to increase with increasing amine content. Concurrently, the connectivity of the softer phase increases dramatically. This two-phase structure is inherently fractal, exhibiting a single glass transition temperature, T-g. The T-g and elevated temperature properties of the epoxy are directly correlated with crosslink density and the percentage of microgel phase observed in microstructure studies. The fracture toughness at room temperature increases with increasing amine content, most likely due to the increased presence of the soft phase, which absorbs more energy during crack growth. Changes in modulus values at 30 degrees C with stoichiometry are explained by considering the effective aspect ratio of the polymer structure in the determination of sample rigidity. Relationships between microgel sizes and the sizes of interphase regions that form in composite and adhesive systems are also discussed in terms of interphase properties.