Chemically treated biologically derived tissues are used extensively in bioprostheses. Unfortunately, while extensive research has focused on chemical treatment technologies to reduce negative in-vivo effects such as mineralization and to enhance overall biocompatiblity, little work has been done on understanding the effects of chemical treatment on tissue mechanical properties. In the current work, a structural constitutive model for chemically treated tissues is presented that seeks to separate out the effects of the chemically treated matrix from that of the collagen fibers. Experimental results from Sacks and Chuong (Sacks and Chuong 1998) using glutaraldehyde bovine pericardium (GLBP) are used to demonstrate the approach. A unique feature of the current approach is the integration of collagen fiber orientation data quantified by small angle light scattering (SALS).