Experiments were performed to study the variation upon storage at room temperature of both the bound rubber and the rheological properties of filled rubber compounds, prepared either with high cis-1,4 polybutadiene or with natural rubber and various levels of a reinforcing carbon black. It was found that the bound rubber and the rheological properties very upon storage according to a two-stage model. Depending on the chemical nature of the rubber, several hundred hours storage are necessary before constant properties are measured. These variations are related to changes occuring in the rubber-black morphology of the compounds. Large strain rheological tests are most sensitive to storage maturation which suggests an explanation for well-known hardening effects observed on the factory floor.