We have compared in vivo spectra of the human forearm, brain, breast and blood-perfluorocarbon exchanged cat brain with reference spectra of deoxyhemoglobin, oxyhemoglobin, triglycerides, water and albumin to identify spectral features related to tissue biochemistry. From these investigations it is evident that bands due to fat, protein and water overlap the deoxyhemoglobin band at 760 nm. While these overlapping absorptions may be insignificant in whole blood, there is a potential for spectral interference in in vivo studies. Furthermore, unique features related the protein, lipids and water can be used to image breast tissue composition. The ability to resolve the overlapping absorption bands appears easily accomplished by using derivative mathematical approaches.