Highly filled rubbers undergo both chemical and physical changes during aging. The most commonly observed changes during ambient aging are in: (1) polymer network structure, (2) gel fraction of the binder, and (3) volumetric loading of solids. A quantitative relationship for the modulus change due to the above factors is developed as follows:Es/Er=(νs/νr)⋅(vs/vr)2⋅[(1−φr)/(1−φs)]3whereEis the Young's modulus of the composite, ν is the crosslink density expressed in moles effective chains per cubic centimeter,vis the gel fraction of the binder, φ is the volume filler fraction of the composite, and subscriptsandrrefer to test specimen and reference material, respectively. Two experiments were performed to evaluate the above relationship: experiment A involved the evaluation of various environmental exposures on thin samples of polybutadiene rubber containing about 76 volume percent solid. Weight loss, swelling ratios, and uniaxial tensile properties were measured. Experiment B involved measurement of the gradient of properties adjacent to the interface between the same filled polybutadiene rubber and lowly filled rubber substrate. Swelling ratios, organic extractables, and uniaxial properties were obtained from thin sections cut from a bulk sample after 18 months storage at 80°F. The experimental data from these studies are used to calculate the reduced modulus,Es/Er,using the above equation. The calculated values agree well with the measured values indicating that the change of modulus is a function of approximately the fifth power of the change in the plasticizer content and approximately the first power of the crosslink density.