Physical aging was used to vary the mechanical properties of model single fiber composites without changing the chemistry at the interface in order to study how property changes affect the measurement of interfacial adhesion by the fragmentation test. The properties of epoxy matrix/AS4 single fiber composites driven to full cure (T-g = 166 degrees C) are altered by annealing below T-g. Neat resin samples with identical thermal histories are tested All aged panels show roughly the same embrittlement with aging characterized by an average 30% decrease in tensile failure strain and 7.3% increase in compressive yield relative to quenched samples. Fragmentation results indicated no change between aged and quenched samples. Results are discussed in terms of micromechanics models for the fragmentation test. Strain at fragmentation increased with aging. This was related to the residual stress state in the model composite and the possibility of the zero stress state of the single fiber composites increasing with thermal annealing.