Aging of soft tissues occurs through various mechanisms, which involve biochemical reactions, like glycation, oxidation and calcification of proteins in the extracellular matrix of the soft tissue. Aging leads to changes in the microstructure of the soft tissues which results in changes in its mechanical behavior. In this paper, a mixture theory based model that is thermodynamically consistent and applicable for chemically reacting systems is developed. The tissue (elastin) is modeled as a viscoelastic material. The model is simulated for uni-axial loading to study the effect of chemical reactions occurring in the tissue (elastin) on its mechanical response. Copyright © 2014 by ASME.